Science@FELS

21 Sept 2026, 08:00 → 25 Sept 2026, 17:00 Europe/Zurich

Auditorium (WHGA/001) (Paul Scherrer Institut)

The international Science@FELs 2026 conference focuses on recent scientific highlights and related research activities from Free Electron Laser (FEL) facilities worldwide. It is organized as a biannual meeting by FELs of EUROPE, the collaboration of European free electron laser (FEL) facilities and advanced short-pulse light sources (SPS). A one and a half day “Forum on Advanced FEL Techniques”, aiming to bring together FEL experts and FEL users, is attached to the main conference and will take place, from 24.-25. September 2026. The Science@FELs conference includes poster sessions, tutorial, invited and contributed talks. 

The conference additionally features the “FELs of EUROPE award on FEL Science and Applications 2026”.  

 

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Science@FELS_2026_Poster_A.pdf

Monday 21 September

08:15 → 08:30 Welcome

08:30 → 10:30 Solid State Physics

Convener: Luc Patthey (PSI - Paul Scherrer Institut)

08:30 Disentangling coherent dynamics in solids with x-rays (Tutorial)

The short wavelength and high photon energy of x-rays offers unique opportunities to selectively probe coherent dynamics in solids. In this talk I will give a brief overview of how x-ray diffraction and absorption spectroscopy can, in combination with selective pump pulses ranging from THz to UV, identify coherent spin and lattice dynamics on different length scales. I will present as an example recent work on coupled spin and lattice dynamics in antiferromagnetic CoF$_2$, as well as a study of GeTe where separate measurements of the dynamics of short- and long-range order can be used to identify the nature of the equilibrium ferroelectric-paraelectric phase transition [1].

[1] Ovuka, V., Toward ferroelectric order control in Germanium Telluride, PhD Thesis, ETH Zurich, 2024

Speaker: Steven Johnson (ETH Zürich)

09:10 Photoexcited transient states in correlated quantum materials investigated by time-resolved RIXS

Resonant inelastic x-ray scattering (RIXS) is a premier tool for the investigation of quantum materials with strong electronic correlation. It is used to study high-energy spin excitations, charge transfer and orbital excitations, charge order and its associated fluctuations, electron-phonon coupling and hybrid modes among the listed ones. At synchrotron x-ray sources, high resolution RIXS has been providing landmark results on cuprates, nickelates and a variety of other transition-metal oxides in the last 20 years. More recently, the advent of high repetition rate x-ray free electron lasers (XFELs) has eventually enabled to add time resolution to RIXS experiments, where the sample is photo-excited by a few femtosecond optical laser pulse and probed by a resonant X-ray pulse. In this context, the realization of the hRIXS instrument at the coherent scattering (SCS) beamline of the European XFEL marks a milestone towards high energy-resolution time-resolved RIXS (trRIXS) [1].
The very first results on NiO and La2CuO4 immediately brought new insight on the photo-doped state of these archetypical charge transfer insulators. In particular, we observed that in NiO the above-gap optical excitation generates a transient charge-transfer excitonic state, whose main signature is an energy gain peak in the transient RIXS. The charge transfer exciton delocalizes within few picoseconds, leaving the system in a metastable state lasting for a much longer time [2].
I will present those pioneering results, and the outcome of successive experiments on NiO and cuprates, excited with different photon energy of the optical pump [3,4]. Although the potential of trRIXS to clarify the nature of photoexcited states in quantum materials is apparent from these early results, the interpretation of the data requires the deployment of an ambitious theoretical effort.

[1] J. Schlappa, G. Ghiringhelli, B.E. Van Kuiken, M. Teichmann, P.S. Miedem., J.T. Delitz, ... & A. Föhlisch, “The Heisenberg-RIXS instrument at the European XFEL. J. Synchrotron Radiation 32, 29 (2025).
[2] Giacomo Merzoni, Leonardo Martinelli, Sergii Parchenko, Sophia FR TenHuisen, Vasily Lebedev, Luigi Adriano, Robert Carley, Natalia Gerasimova, Laurent Mercadier, Martin Teichmann, Benjamin E van Kuiken, Zhong Yin, Amina Alic, Denitsa R Baykusheva, Sorin G Chiuzbaian, Stefano Dal Conte, Oleg Dogadov, Alexander Föhlisch, Maurits W Haverkort, Maximilian Kusch, Tim Laarmann, Wei Sheng Lee, Marco Moretti Sala, Ying Ying Peng, Qing Zheng Qiu, Thorsten Schmitt, Sreeju Sreekantan Nair Lalithambika, Simone Techert, Giulio Cerullo, Michael Först, Matteo Mitrano, Mark PM Dean, Justine Schlappa, Andreas Scherz, Giacomo Ghiringhelli “Photo-generated charge-transfer excitons in NiO revealed by time-resolved resonant inelastic x-ray Scattering”, arXiv:2504.16653.
[3] Daniel Jost, Jiarui Li, Jordyn Hales, Jonathan Sobota, Giacomo Merzoni, Leonardo Martinelli, Shuhan Ding, Kejun Xu, Justine Schlappa, Andreas Scherz, Robert Carley, Benjamin E Van Kuiken, Teguh C Asmara, Le Phuong Hoang, Laurent Mercadier, Sergii Parchenko, Martin Teichmann, Patrick S Kirchmann, Giacomo Ghiringhelli, Brian Moritz, Zhi-Xun Shen, Thomas P Devereaux, Yao Wang, Wei-Sheng Lee, “Robust Paramagnon and Acoustic Plasmon in a Photo-excited Electron-doped Cuprate Superconductor”, arXiv:2511.22054
[4] Giacomo Merzoni, Laurent Mercadier, Teguh Citra Asmara, Le Phuong Hoang, Sergii Parchenko, Eugenio Paris, Benjamin E. Van Kuiken, Floriana Lombardi, Robert Carley Riccardo Arpaia, Andreas Scherz, and Giacomo Ghiringhelli, “In-gap optical quenching of charge order in YBa2Cu3O7−δ”, 2026 unpublished

Speaker: Giacomo Ghiringhelli (Politecnico di Milano)

09:35 Collective Magnetic Excitations in a Photo-excited Electron-doped Cuprate Superconductor

Unravelling the response of microscopic degrees of freedom in photoexcited cuprates is a central challenge in ultrafast materials science. While the charge response has been extensively investigated via time-resolved (tr-) optical spectroscopy and tr-ARPES, the nonequilibrium dynamics of magnetic degrees of freedom remain largely unexplored due to the lack of a direct probe. The advent of high-resolution RIXS instrument at high-repetition-rate X-ray FELs now enables high-quality tr-RIXS measurements at the Cu L-edge, providing a window into the underlying magnetic excitations. In this presentation, I will discuss our tr-RIXS experiments on the photoexcited electron-doped cuprate Nd2-xCexCuO4. We track the light-driven response of the paramagnon as a function of time and momentum, revealing a transient modification of the dispersion and a spectral weight redistribution within the Brillouin zone. These results highlight the intriguing prospects of manipulating magnetic degrees of freedom via light-matter interactions in quantum materials.

Speaker: Wei-Sheng Lee (SLAC National Accelerator Lab.)

Science_FEL_2026_WSL_Final.pdf

Science_FEL_2026_WSL_Final.pptx

10:00 Ultrafast phase transition of silicon monitored by soft x-ray absorption spectroscopy

Silicon undergoes a sub-picosecond non-thermal melting transition when excited by intense femtosecond laser pulses. This talk demonstrates how soft X-ray absorption spectroscopy (XAS), performed at the seeded FERMI free-electron laser (FEL), effectively captures this process by resolving the ultrafast electronic structure dynamics of silicon with a temporal resolution on the 10 fs timescale.

By tuning the FERMI FEL photon energy across the Si L₂,₃-edge following femtosecond UV excitation, we reveal a sub-picosecond rearrangement of the electronic structure. This excitation induces a rapid destabilization of the covalent bonds, leading to a lattice collapse before significant phonon-driven thermal effects can take place. The observed dynamics of the XAS spectrum, associated with bandgap collapse and electronic redistribution, provide direct evidence of a non-thermal melting transition.

Speaker: Zeinab Ebrahimpour (Elettra Sincrotrone Trieste S.C.p.A.)

10:15 Escaping electronic damage using ultrafast X-ray pulses

Diffraction-before-destruction schemes rely on collecting data before radiation damage develops. This condition is readily satisfied for atomic motion, making serial femtosecond crystallography highly successful in investigating photoinduced structural changes in both organic and inorganic matter. It is less clear, however, whether the condition also holds for the electronic contributions to the structure factors. Using broadband, temporally chirped X-ray pulses, achieving few-fs resolution, we compare the structurally forbidden reflections of elemental silicon, which are predominantly sensitive to valence electron density, with simultaneously measured allowed lattice Bragg peaks. We observe photoinduced valence bond depletion on sub-10-fs timescales, well before significant lattice disorder takes place. Our results establish the temporal limits of diffraction-before-destruction of electronic orders and provide a new route to disentangling electronic and structural dynamics in crystalline solids at few to sub-femtosecond timescales.

Speaker: Jakub Vonka (PSI - Paul Scherrer Institut)

10:30 → 11:00 Coffee break

11:00 → 13:00 AMO Physics

Convener: Kirsten Andrea Schnorr (PSI - Paul Scherrer Institut)

11:00 Element-specific measurements of attosecond dynamics in isolated and solvated molecules

The recent capability of creating soft-X-ray attosecond pulses at free-electron lasers has opened the possibility of performing element-specific measurements of electronic dynamics on their natural time scale. I will present recent results from four user beam times at LCLS. Combining attosecond soft-X-ray pulses and circularly polarized infrared pulses, we have measured attosecond photoionization delays between the N1s and C1s shells of azabenzenes (pyridine, pyrazine, s-triazine) [1]. This study demonstrates the unique advantages of core-level chronoscopy, which make the technique scalable to complex systems: the photoelectron spectra remain atomic-like, the measurements are element specific and the photoelectron waves originate from a point-like source. This study moreover identified two unknown mechanisms regulating photoionization delays in molecules: (i) the substitution of C-H groups with N atoms increases the confinement of shape resonances and (ii) the increasing symmetry decreases the coupling between photoelectron partial waves. Both mechanisms lead to an increase of the photoionization delays, which was experimentally observed. In the second beam time, we demonstrated X-ray photoelectron spectroscopy in an attosecond pump, attosecond probe setting. This new technique was used to observe the recently proposed core-hole coherent spectroscopy [2]. Preparing coherent superpositions of core-hole states in N2O and 2-nitropyridine, we have observed electronic dynamics with periods of 1.1 and 0.83 fs, respectively [3]. Remarkably, the decoherence of the electronic wave packet is faster in the case of N2O than in 2-nitropyridine, which reveals the different topologies of the respective potential-energy surfaces. In the third beam time, we demonstrated attosecond double-resonant core-hole spectroscopy. Photoexciting nickelocene at the carbon K-edge, probing it at the nickel L-edge and detecting the Auger electrons, we identified the signatures of nickel core-hole screening and electronic coherences on sub-femtosecond time scales [4]. In the fourth beam time, attosecond pump, attosecond probe measurements have been performed on aqueous urea solutions. The ionization-induced dynamics of urea solutions have been probed by time-resolved X-ray absorption spectroscopy at the oxygen K-edge. These results demonstrate the considerable potential of attosecond spectroscopy in the soft-X-ray domain, illustrating the key advantages of element- and site-specific probing.

[1] J.-B. Ji, Z. Guo et al., Phys. Rev. X 15, 041031 (2025)
[2] E. Rodriguez-Cuenca et al., Phys. Rev. Lett. (2024)
[3] F. Vismarra et al., in preparation
[4] F. Vismarra et al., in preparation
[5] S. Lee et al., in preparation

Speaker: Hans Jakob Wörner (ETH Zürich)

11:25 Imaging collective quantum fluctuations of a complex molecule using Coulomb explosion

Heisenberg uncertainty principle does not allow atoms in a molecule to have perfectly well-defined positions. Thus, each measurement of a molecular geometry probes a slightly different structure, even in the ground state. These quantum fluctuations are collective in essence: the atoms do not fluctuate independently relative to each other, but follow concerted patterns, usually described by normal modes. Measuring them in a complex molecule is challenging, as it requires a method that captures structural information from measurements of single molecules.

In this talk, I will present how we achieved it for an 11-atom molecule by inducing its Coulomb explosion at the European XFEL. In the experiment, the momenta of the fragments are measured in coincidence, and I will show that this data carries information about the ground-state fluctuations of the molecular structure despite our measurements detecting only a fraction of the 11 atomic fragments in coincidence. I will describe how we tackled this challenge by developing an analysis method that reconstructs the missing information. We use it to then assign specific features of the measured momenta to specific structural fluctuations of the molecule before its explosion, establishing that Coulomb explosion has the power to probe collective molecular effects in large molecules.

Speaker: Benoît Richard (Hamburg Center for Ultrafast Imaging / EuXFEL)

11:50 Mapping XFEL Nanofoci with X-ray Fluorescence Intensity Correlations

When X-ray fluorescence is excited by X-ray pulses with durations approaching the coherence time of the fluorescence, it encodes spatial information about the structure of the fluorescing volume and temporal information about the excitation pulse. Both spatial and temporal information can be revealed by analyzing intensity correlations in the emitted fluorescence, a method sometimes referred to as “incoherent diffractive imaging”.

In this contribution, we report results from our recent measurements of X-ray fluorescence intensity correlations at the Linac Coherent Light Source (LCLS) using thin vanadium foils and sub-femtosecond hard X-ray pulses.
We discuss the amount of information about focus shape and intensity distribution that can be obtained from a single intense shot, and show that intensity correlations could provide a promising route to a shot-resolved nanofocus diagnostics.
In addition, we use the correlation contrast to estimate the mean pulse duration, and present experimental data showing distinct projections of the nanofocus from slightly different angles, recorded on a single detector without the need for rotation.

Speaker: Robert Radloff (Institut für Experimentalphysik, Universität Hamburg)

12:05 Controlling ionization with shaped FEL-pulses

Rabi oscillations in resonantly driven two-level systems produce an Autler-Townes doublet, whose components can be selectively addressed with chirped pulses and probed via photo-electron spectra. We previously showed 1 that the ionization step depends strongly on the chirp direction — unlike rapid adiabatic passage, where near-complete population inversion occurs regardless of chirp sign. This asymmetry enables absolute control over ionization, allowing the chirp alone to switch a Helium gas between zero and full ionization.

This prediction motivated an experiment by Lukas Bruder's group (Albert-Ludwigs-Universität Freiburg) at the FERMI free-electron laser (Trieste), one of the first quantum-control studies using shaped pulses at such a facility. Targeting the 1st excited state of Helium with circularly polarized pulses ensured strong, clean coupling and a tractable theoretical picture. By propagating the TDSE for a single-active-electron model of Helium, we fully reproduced the measured data, con-firming that chirp-based absolute control of ionization works as predicted 2.

We also report on the response to pulses in which an attenuator was used to cut a "notch" into the pulse spectrum right at the Helium resonance.

1 U Saalmann, S K Giri, and J M Rost, Adiabatic passage to the continuum: Controlling ionization with chirped laser pulses. Phys. Rev. Lett. 121, 153203 (2018).

2 F Richter, U Saalmann, E Allaria, M Wollenhaupt, B Ardini, A Brynes, C Callegari, G Cerullo, M Danailov, A Demidovich, K Dulitz, R Feifel, M D Fraia, S D Ganeshamandiram, L Giannessi, N Gölz, S Hartweg, B von Issendorff, T Laarmann, F Landmesser, Y Li, M Manfredda, C Manzoni, M Michelbach, A Morlok, M Mudrich, A Ngai, I Nikolov, N Pal, F Pannek, G Penco, O Plekan, K C Prince, G Sansone, A Simoncig, F Stienkemeier, R J Squibb, P Susnjar, M Trovo, D Uhl, B Wouterlood, M Zangrando, and L Bruder, Strong-field quantum control in the extreme ultraviolet domain using pulse shaping. Nature 636, 337 (2024).

Speaker: Ulf Saalmann (Max Planck Institute for the Physics of Complex Systems)

12:20 Probing IR-induced Ultrafast Plasma Dynamics in Doped Helium Nanodroplets by Time Resolved X-ray Photoelectron Spectroscopy

The investigation of light-matter interaction with weakly-bound van der Waals clusters of atoms bridges the gap between the photophysics of isolated atoms and bulk materials. When exposed to intense ultrashort laser pulses, these clusters can efficiently absorb the energy, and transform into dense, short-lived plasma. This leads to the emission of energetic electrons and highly charged ions, and even X-rays and neutrons, with potential applications ranging from producing attosecond pulses via High Harmonic Generation (HHG) to triggering nuclear fusion [1]. Consequently, studying these plasma states is fundamental to uncovering the complex many-body physics of matter under extreme conditions.

We studied how these plasmas form and evolve with time in the prototype system of helium nanodroplets doped with argon atoms upon irradiation of intense infrared (IR) femtosecond pulses [2]. Ultrashort soft X-ray pulses from SwissFEL were used to further ionize the system at different time delays with respect to the IR pulses, and corresponding X-ray photoelectron spectra (XPS) were recorded using a hemispherical energy analyser at the Maloja endstation [3]. Here, for the first time, we successfully demonstrated that the recorded time resolved XPS maps can capture the evolution of Coulombic cluster potential of the IR-induced nanoplasma, which perturbs the electron binding energies of the constituents, in real time up to a few nanoseconds. Insights gained from these spectroscopic data, such as, formation of He+ and He2+ ions and their subsequent recombination with electrons inside the plasma, will be presented in detail, alongside supporting theoretical calculations. Furthermore, the time resolved XPS scheme implemented here serves as a more direct probe for characterizing the evolution of the mean-field cluster potential and the underlying ion-electron recombination dynamics during plasma expansion, in contrast to other experimental methods, such as, fluorescence measurements, time resolved- ion time of flight and Auger electron spectroscopy [4].

References:
[1] Th. Fennel et. al., Rev. Mod. Phys., 82, 2 (2010), U Saalmann et. el., J. Phys. B: At. Mol. Opt. Phys. 39 (2006) R39–R77 [2] S R Krishnan et. al., Phys. Rev. Lett. 107, 173402 (2011), S R Krishnan et. al., New Journal of Physics 14 (2012) 075016 [3] E. Prat et. al., Nature communications, 2023, 14, 5069. R. Abela et. al., Synchrotron Radiation, 2019, 26, 1073–1084 [4] S. M. Vinko et. al., Nature, 482, 59–62 (2012), M. Kelbg et. al., Phys. Rev. Lett., 125, 093202 (2020)

Speaker: Suddhasattwa Mandal (Paul Scherrer Institut PSI)

12:35 Femtosecond FEL-induced dynamics: from 2-level system to complex systems

Free-electron lasers (FELs) providing coherent light pulses ranging from mid-infrared to hard X-ray domain have opened up novel ways to study fundamental aspects of light-matter interactions. The ultrashort (10$^{-15}$ s to 10$^{-18}$ s) and ultra-intense (>10$^{13}$ W/cm$^2$) nature of these pulses allow probing the coherent nature of these interactions. One such example is the Rabi oscillation [1] $-$ the periodic transfer of population between two levels driven by an external electromagnetic field. While these oscillations have been studied mainly in the long wavelength regime, such as from infrared to visible, using femtosecond extreme ultraviolet (XUV) pulses from the seeded FEL FERMI [2], we could study Rabi oscillations in bound-bound transitions for neutral helium [3] as well as for singly-charged helium ion [4]. For the latter case, the high intensity of the FEL pulse allows us to create quantum entanglement between the emitted photoelectron and the XUV-dressed He$^+$ ion undergoing Rabi oscillations. The emitted photoelectron wave-packet entangled with the He$^+$-ion expands to a distance of few hundred nanometers, thus providing indirect evidence of quantum entanglement at mesoscopic length-scale [4]. While seeded FELs are known for their superior temporal coherence properties, they mostly operate in the XUV domain. Compared to that, SASE FELs, despite their low temporal coherence, can go high up in photon energies allowing us to probe ultrafast dynamics in complex molecular systems close to different K$_{\alpha}$-edges. We used a UV-pump $-$ soft X-ray-probe scheme at the European XFEL [5] to study ring opening and subsequent isomerization processes inside a photo-switch molecule in gas phase. The time-dependent mass spectra as well as the X-ray photoelectron spectra show the signature of an ultrafast (~500 fs) conversion from one isomer to another, following excitation of the neutral molecule to a highly-excited state [6].

References:
[1] I.I. Rabi, Space quantization in a gyrating magnetic field. Phys. Rev. 51, 652-654 (1937)
[2] E. Allaria et al., Highly coherent and stable pulses from the FERMI seeded free-electron laser in the extreme ultraviolet. Nat. Photon. 6, 699-704 (2012)
[3] S. Nandi et al., Observation of Rabi dynamics with a short-wavelength free-electron laser. Nature 608, 488-493 (2022)
[4] S. Nandi et al., Generation of entanglement using a short-wavelength seeded free-electron laser. Sci. Adv. 10, eado0668 (2024)
[5] W. Decking et al., A MHz-repetition-rate hard X-ray free-electron laser driven by a superconducting linear accelerator. Nat. Photon. 14, 391-397 (2020)
[6] S. Nandi et al., under preparation (2026).

Speaker: Saikat Nandi (Institut Lumière Matière, CNRS)

13:00 → 14:00 Lunch

14:00 → 16:00 Chemistry

Convener: Claudio Cirelli (PSI - Paul Scherrer Institut)

14:00 Testing the predictivity of computational photochemistry with ultrafast experiments

Can nonadiabatic molecular dynamics simulations predict a photochemical experiment before it is performed? I will discuss this question through the recent cyclobutanone prediction challenge, in which several theory groups predicted the photochemistry and ultrafast electron diffraction signal following 200 nm excitation. The results showed that current methods can often capture the main qualitative dynamics, but that quantitative predictions remain sensitive to electronic structure, initial conditions, nonadiabatic dynamics, and how experimental observables are simulated. The challenge illustrates how ultrafast experiments can test theory not only by providing signals to interpret, but by confronting simulations with observables predicted before the data are known. Such tests would be strengthened by multimodal experiments, where complementary probes separate structural, electronic, and product-channel information, making it possible to identify more precisely what is missing from current models of photochemical dynamics.

Speaker: Nanna Holmgaard List (KTH Royal Institute of Technology; University of Birmingham)

14:25 Time-Resolved X-ray Photoelectron Spectroscopy of 200 nm-Excited Cyclobutanone

Photodissociation of cyclobutanone following 200 nm excitation has been extensively studied as a model system for nonadiabatic dynamics in cyclic ketones. Previous studies have shown that excitation populates the S₂ state, followed by rapid internal conversion through S₁ to the ground state S0 via conical intersections. The resulting vibrationally hot ground-state molecules undergo ring opening and fragmentation, producing CO and ketene. While ultrafast optical spectroscopy and electron diffraction have clarified the structural evolution along this pathway, direct tracking of the electronic structure changes has remained limited.
Here, we present time-resolved X-ray photoelectron spectroscopy (TR-XPS) measurements of gas-phase cyclobutanone following 200 nm excitation at the SwissFEL Athos beamline Maloja endstation. The measurements achieve an instrument response of 53 fs, enabling direct observation of the early relaxation dynamics. Figure 1 shows the TR-XPS map around the O 1s ionization region. The spectra reveal a pronounced ground-state bleach at 537.9 eV, the emergence of a transient excited-state feature at 541.2 eV, and the subsequent appearance of spectral signatures assigned to the ketene (539.9 eV) and CO (542.4 eV) dissociation channels.
To aid the interpretation of the transient spectra, quantum chemical calculations and complementary TR-XPS measurements at the C 1s edge were performed and will be discussed to provide further insight into the redistribution of electronic structure during relaxation and fragmentation. These results demonstrate the capability of femtosecond TR-XPS to simultaneously monitor electronic relaxation and competing dissociation pathways with atomic-site specificity. The combination of time-resolved core-level spectroscopy and quantum chemical simulations provides new insights into the nonadiabatic dynamics governing the photochemistry of cyclobutanone.

Speaker: Dr Xinhua Xie (PSI - Paul Scherrer Institut)

14:50 Electronic precursors of proton transfer in urea solution revealed by attosecond X-ray pump and X-ray probe spectroscopy

A central tenet of attochemistry is charge-directed reactivity: ionization launches a valence hole whose motion and eventual localization across a molecule can predetermine the ensuing dynamics [1]. Yet, despite successes in capturing attosecond electron dynamics, their causal link to reaction dynamics, particularly in liquid solutions, has remained experimentally elusive. Concentrated aqueous urea is an ideal model for this link, as it assembles into hydrogen-bonded urea dimers where valence ionization selectively drives an ultrafast proton transfer within the dimer, while the surrounding water remains a spectator. Table-top carbon K-edge transient absorption spectroscopy has revealed the femtosecond proton transfer dynamics [2], but the attosecond-to-few-femtosecond electronic precursors preceding it have remained unveiled.

Here we employ all-X-ray attosecond transient absorption spectroscopy (AX-ATAS) to capture the electron dynamics within the urea dimer directly. Using the ω/2ω XLEAP setup at LCLS, an ω pulse valence-ionizes the sample while a 2ω attosecond pulse probes the oxygen K-edge valence hole [3]. We also integrated a covariance-based single-shot spectral reconstruction [4] to make an improved spectral resolution as benchmarked in pure liquid water.

In ionized urea solution, we observed a distinct transient absorption feature near 526 eV, clearly separated from the nearby water valence-hole response and from the two previously characterized urea bands. It rises within the first few femtoseconds and evolves differently from the known urea peaks near the absorption edge. This is consistent with the 1s to HOMO transition of oxygen predicted by QM/MM simulations to track the structurally driven oscillation of the valence hole between the two urea moieties, the fingerprint anticipated for charge-directed reactivity. To our knowledge, this offers the first experimental signature of the ultrafast valence-hole dynamics that direct proton transfer in a hydrogen-bonded liquid, connecting electron precursors to the proton transfer observed previously at the carbon K-edge [2] and in liquid water [3].

References
[1] F. Remacle et al., Chem. Phys. Lett. 285, 25 (1998)
[2] Z. Yin et al., Nature 619, 749 (2023)
[3] S. Li et al., Science 383, 1118 (2024)
[4] T. Driver et al., Phys. Chem. Chem. Phys. 22 (5), 2704-2712 (2020)

Speaker: Seonggon Lee (ETH Zurich)

15:05 Unveiling the Charge Carrier Dynamics in Colloidal Quantum Dots using Ultrafast X-ray and Optical Spectroscopies.

Semiconductor CuInS$_2$ (CIS) quantum dots (QDs) are highly promising for optoelectronic applications for their low toxicity, excellent charge-transport properties, and remarkable photoluminescence features. To develop our fundamental understanding of nanoscale photo-physics and optimize the design of devices, several mechanisms of recombination in CIS QDs have been proposed, modelling the population dynamics of correlated electron-hole pairs (bound excitons). While most suggest that radiative recombination results from the interaction between an electron in the conduction band and a hole in the so-called confined hole state (CHS), no research has provided direct experimental evidence for the existence of this intra-gap state. The existence of such a state, arising from a defect of Cu$^+$ that can get oxidized to Cu$^{2+}$ by the hole, would explain the large Stokes shift and broad photoluminescence exhibited by the CIS QDs and has been theoretically predicted and postulated for a long time. In this study, we aim to observe and understand the formation process of the CHS, followed by the localization of a hole in the CHS. The novelty of this project stems from the experimental methodologies employed to tackle this problem: we use a combination of ultrafast optical and X-ray free electron laser (XFEL) pump-probe techniques on tunned CIS QDs. This approach allows us to observe the spectral fingerprints of photogenerated charge carriers directly on the fundamental timescales of their formation and relaxation within a QD but also, with atomic spatial resolution provided by hard X-rays delivered by an XFEL. Moreover, the element and oxidation state specificity of X-ray, serves as a direct probe to track recombination and localization dynamics of photogenerated holes. Furthermore, the femtosecond-resolved optical studies complement our results by monitoring of the photoinduced dynamics of photoelectrons. This extended study grants new insights into this class of materials and correlates the observed charge carrier dynamics, influenced by the laser fluence.

Speaker: Elisa Pauline Pascale Collet (IMDEA Nanoscience)

15:20 Characterizing Proton-Coupled Electron Transfer Reactions using Time-Resolved X-ray Photoelectron Spectroscopy

Proton-coupled electron transfer (PCET) is a fundamental class of photochemical reactions in which the absorption of light triggers both charge transfer and a change in hydrogen bonding on the excited state potential surface in molecules where a hydrogen donor and acceptor group are in close proximity to one another. The distinct proton and electron transfer steps may occur sequentially or concomitantly and the process itself as well as the nature of the resulting product species are often intimately linked to the coupling of electronic and nuclear degrees of freedom [1, 2]. A full characterization of a PCET reaction involves distinguishing which step happens first, how much partial charge is transferred during each step, and which nuclear motions are necessary to complete the reaction.

In order to answer these questions, a femtosecond probe sensitive to both the electronic and nuclear structure is required. Time-resolved X-ray photoelectron spectroscopy (TR-XPS), now possible at the current generation X-ray free electron lasers, represents an ideal probe for such dynamics because it combines the ultrafast time resolution and inherent element-selectivity of femtosecond X-ray pulses with the sensitivity of core-level XPS to the local chemical environment of specific sites within the system.

In this talk, I will present results from a TR-XPS measurement on the prototypical PCET system 2-(2′-hydroxyphenyl) benzothiazole in the gas phase performed at the Maloja endstation of SwissFEL’s soft X-ray beamline. By combining experimental observations at the donor and acceptor sites with state-of-the-art core-level XPS calculations, the charge transfer and proton transfer steps are unambiguously distinguished and the partial charges involved in the reaction are extracted. By analyzing the nuclear wavepacket dynamics in the excited state, the nuclear modes active in the PCET process are identified. Finally, the formation of two distinct and long-lived product states via a conical intersection is observed by tracking the chemical shifts as the molecule undergoes its proceeding nonadiabatic dynamics.

[1] S. Lochbrunner et al., The Journal of Physical Chemistry A, 107, 49, 10580–10590 (2003).
[2] S. Pijeau et al., The Journal of Physical Chemistry A, 121, 24, 4595-4605 (2017).

Speaker: Antoine Sarracini (PSI - Paul Scherrer Institut)

16:00 → 20:00 Apero@SWISSFEL

16:00 → 20:00 Poster Session (I)

Convener: ALL Posters Science@FELs

16:00 Commissioning of TRESOR - A High Resolution Electron Spectrometer at the SQS Instrument of euXFEL

A Scienta EW4000-6kV hemispherical analyzer has been commissioned at the SQS instrument of the European XFEL. Conceived as a high-resolution electron spectrometer for gas phase studies, TRESOR was installed and first used at euXFEL in spring 2026. With a delay line anode on the detector, the spectrometer can take advantage of the high repetition rate available at euXFEL to overcome problems with space charge that generally limit the utility of this type of analyzer at XFELs while still collecting pulse-by-pulse data for subsequent sorting. In the first runs with the instrument we demonstrated <100 meV resolution on rare gas valence lines and vibrational resolution of inner-shell electron spectra of small molecules CO, N2, CH3F. We further showed that the analyzer can be used with the full SASE beam with pulse energies up to several mJ to give high resolution spectra of multiphoton process such as double core hole creation and decay. First results will be presented showing the suitability of TRESOR for high-resolution electron spectroscopy studies at euXFEL.

Speaker: John Bozek (Synchrotron SOLEIL)

16:01 Tracing the X-ray induced dynamics in water and ethanol molecules

Abstract: Understanding X-ray induced dynamics is relevant for a broad field of applications, ranging from X-ray imaging of materials, to medical application of X-rays. Due to the large amount of energy imparted by an absorbed X-ray photon, the triggered relaxation processes typically take place on ultrafast time scales. Water molecules in the gas phase have been studied at the Maloja endstation using an X-ray pump X-ray probe scheme. The pump pulse was tuned to 390eV, which leads to valence (1b1, 3a1, 1b2) and inner-valence (2a1) ionization of the molecules, but favors innervalence ionization. Upon ionization of the two outermost orbitals, we expect stable cations, while ionization of the 1b2 and 2a1 orbitals is expected to result in dissociation. We probe the cations and dissociation products using X-ray absorption spectroscopy (XAS) by scanning the photon energy across the oxygen K-edge. The charge-state specific ion yields are detected with an ion time-of-flight (TOF) spectrometer. The resulting time-resolved ion-yield XAS spectra track the electronic structure of the fragments produced upon ionization of water as they undergo further dissociation. In this way, ion TOF spectroscopy captures both the initial excitation and the subsequent fragmentation pathways in a single measurement.1[2] The figure shows a 2D difference map of the evolution in time (y-axis) and probe photon energy (x-axis) of the O+ ion yield. The blue negative features correspond to the ground-state bleach of the H2O resonances at 534-540eV, while two strong positive signals can be observed arising on different timescales. The one around 532eV is assigned to O2. While the other shifts towards lower energies and contains resonances tentatively assigned to H2O+ and OH. Insights into water fragmentation obtained from the spectroscopic data, complemented by simulations performed by our theory collaborators, will be presented. These results provide a benchmark for the subsequent analysis of water clusters recorded during the same beamtime.
Furthermore, we compare X-ray induced dynamics in ethanol molecules and clusters, studied with an X-ray pump NIR probe scheme, to understand how the environment affects the relaxation processes. The pump pulse, tuned above the oxygen K-edge, doubly ionizes ethanol. In isolated molecules, intramolecular hydrogen mobility gives rise to a roaming H2 molecule, one of the channels for H3+ production [3], which we track with time-resolved ion yields. In clusters, both intramolecular and intermolecular proton transfer occur, producing characteristic fragments such as H3O+. Further analysis will allow us to infer the timescales of the relaxation processes in the clusters. This experiment deepens our understanding of the interplay between local and non-local relaxation processes and their consequences for the fragmentation dynamics. 1 M. N. Piancastelli et al., Phys. Rev. A 59.1 (Jan. 1999), pp. 300–306. [2] Julius Schwarz et al., Phys. Chem. Chem. Phys. 24 (38 2022), pp.
[3] Ngai et al. Sci. Rep. 15, 3201 (2025) 23119–23127.

Speaker: Eloisa Manetti (PSI - Paul Scherrer Institut)

16:02 Submicron-Precision Machining of Silicon Surface with Intense Femtosecond Extreme Ultraviolet Pulses

Femtosecond laser direct writing (LDW) using ultraviolet, visible or infrared lasers is a maskless subtractive manufacturing approach that is widely used for precision surface processing due to the strong spatial confinement of laser energy it provides and the minimal collateral damage it causes. Here, we present a conceptual scheme for high-precision machining of semiconductor materials using LDW with sub-micron-focused extreme ultraviolet (EUV) laser pulses. A proof-of-concept study was conducted at the SACLA free-electron laser facility, confirming the feasibility of the machining by creating reproducible craters with similar geometries in single EUV laser shots under fixed pulse parameters. The trends observed in the experiment were reproduced by theoretical simulations. These results clearly demonstrate the potential for controlled and reproducible semiconductor surface machining at the sub-micron length scale [1]. The proposed EUV LDW approach could pioneer transformative, cost-efficient manufacturing technology enabling direct, sub-micron- to nanometer-scale patterning of semiconductors and other technologically relevant materials.
[1] V. Lipp, G. Yamaguchi, S. Egawa et al., submitted to Scientific Reports.

Speaker: Dr Vladimir Lipp (European XFEL)

16:03 Status and future prospects of FHI FEL: A two-color dual-oscillator infrared free-electron laser

We report on the design, performance and applications of FHI FEL, a dual-oscillator two-color infrared free-electron laser which we operate at the Fritz Haber Institute (FHI) in Berlin, Germany [1]. It includes a 50 MeV normal-conducting linac supplying two oscillator FEL branches; a mid-infrared (MIR) FEL (commissioned in 2013) and a new far-infrared (FIR) FEL. What makes the FHI FEL unique among present and previous FELs worldwide is the two-color mode of simultaneous operation of two oscillator FELs powered by a single accelerator. In this mode the high-repetition rate (1 GHz) electron bunch train provided by the accelerator is split by a transverse deflection kicker cavity into two bunch trains, each one having half the repetition rate (500 MHz). The half-repetition-rate bunch trains are separated by an angle of 4 degree and steered into the MIR and FIR FELs. The lasing wavelengths of both FELs can be tuned independently from one another by undulator gap variation. This enables continuous variation of the MIR-to-FIR wavelengths ratio by as much as a factor of 10. We provide experimental evidence indicating that the MIR and FIR radiation pulses are highly synchronized and, hence, well suited for a wealth of novel applications that have not been possible before such as, pump-probe experiments.

The FIR FEL was designed as a short-Rayleigh-range oscillator FEL, thereby optimizing the achievable micro-pulse energy [1]. In a recent line of experiments we were able to boost the micro-pulse energy to more than 150 µJ at a wavelength of 8 µm when running the FIR FEL at a small cavity-length detuning of less than a wavelength. At these conditions we observed a very wide spectrum indicating a short (sub-ps) pulse duration, as expected for small FEL cavity desynchronism. Upon tightly focusing these pulses into a Xe gas we observed ionization of Xe atoms, which is remarkable considering that the Xe atom’s ionization energy (12.1 eV) equals about 78 times the photon energy at 8 µm. We interpret this observation as evidence that electric fields strengths strong enough to cause tunneling ionization of atoms can be achieved. This, in principle, opens up the possibility to observe HHG (high-order harmonic generation). Experiments to explore the feasibility of generating ultra-short XUV pulses by HHG using the FHI FIR FEL are being prepared.

[1] W. Schöllkopf et al., arXiv:2604.16189

Speaker: Wieland Schöllkopf (Fritz-Haber-Institut der Max-Planck-Gesellschaft)

16:04 NIR-laser induced dressing signatures in helium nanodroplets probed by coherent diffractive imaging

Single-shot coherent diffractive imaging (CDI) enables the capture of a full diffraction image of a nanostructure using a single flash of XUV or X-ray light. The resulting scattering image encodes both the geometry and the optical properties of the target. So far, this method has mainly been employed for ultrafast structural characterization [1]. However, CDI can also be utilized to resolve ultrafast optical property changes caused by e.g. transient excitation from nonlinear scattering [2], or by illumination with a second ultra-short laser pulse.

Here, we explore the expected signatures for the latter case theoretically, where simultaneous exposure to a strong IR field can induce transient optical properties. To this end, the effective optical properties, emerging from the laser dressing, are extracted from the dipole response of a local quantum description based on an atom-like solution of the time-dependent Schrödinger equation. In a second step, we apply the well-known Mie-solution, to use the obtained optical properties in order to describe the resulting scattering process at helium nanodroplets and compare the results to recent experiments [3].

[1] I. Barke et al., Nat. Commun. 6, 6187 (2015)

[2] B. Kruse et al., J. Phys.: Photonics 2, 024007 (2020)

[3] J. Schäfer-Zimmermann et al., Ultrafast Sci. 6, 0146 (2026)

Speaker: Tom von Scheven (University of Rostock)

16:05 Modeling Laser-induced Dynamics and Material Modification in Thin Gold Foils

Understanding the dynamics underlying the interaction of ultrashort intense laser pulses with metals is of key interest for advancing controlled laser-based material processing. The irradiation of metal targets with intense laser pulses triggers a variety of physical processes, including the initial laser heating of the electrons, energy transfer between the electronic and lattice system and eventually ionic ablation from the target. The induced structural changes can be characterized experimentally via pump-probe coherent diffractive imaging (CDI).

We have developed a multistage approach to model the ablation of a thin gold foil from the excitation to the expansion phase using a two-temperature model and subsequent molecular-dynamics simulations. To translate the evolving structures into experimentally accessible observables we derived the transient optical properties of the sample during ablation by adapting the parameters of the well-known Drude-Lorentz model with density- and temperature dependent electronic configurations from an average-atom model. In a second step we simulated time-resolved diffraction patterns using the adapted optical properties and the recently developed pMSFT method and compared the results with CDI measurements.

Speaker: Maura Münier (Universität Rostock)

16:06 Ultrafast Carrier Dynamics in CuO/TiO2 p–n Heterostructures Probed by Time-Resolved Soft X-ray Absorption Spectroscopy

Understanding charge carrier dynamics at p–n heterojunctions is essential for designing high-efficiency photo-catalysts. Here, we investigate ultrafast charge transfer pathways in a layered CuO/TiO2 heterostructure using time-resolved soft X-ray absorption spectroscopy (tr-XAS) with PAL-XFEL for probing transient electronic structure with elemental and orbital sensitivity.
By tracking transient signals at the O K-, Cu L3-, and Ti L2-edges, we identify a pronounced hole accumulation in the CuO valence band on a nanosecond timescale. In contrast, the photo excited electron exhibits a multistep evolution across the junction. Immediately after excitation, electrons in the TiO2 conduction band diffuse into the CuO conduction band within approximately 150 ps. Subsequently, these electrons drift back toward the TiO2 conduction band within about 800 ps, consistent with favorable energetic alignment (lower-lying TiO2 CB) and the larger electron population reservoir in TiO2.
Notably, electrons residing in TiO2 persist up to ~3500 ps, attributable to trapping into deep trap states. This spatial charge separation, driven by the built-in electric field at the p–n interface, effectively suppresses electron–hole recombination and stabilizes long-lived carriers. These findings highlight the power of element-resolved ultrafast X-ray spectroscopy for disentangling interfacial carrier pathways and motivate future XFEL based femtosecond-to-nanosecond studies of heterojunction photo catalysts.

Speaker: Sang Han Park (Pohang Accelerator Laboratory, Pohang University of Science and Technology (POSTECH))

16:07 Electron-positron pair beams enable beyond-terawatt attosecond free-electron lasers from soft X-rays to coherent gamma-ray emission

Free-electron lasers (FELs) produce intense, coherent light pulses spanning the infrared to hard X-rays, enabling ultrafast studies of matter on atomic scales. Extending FEL operation toward gamma-ray photon energies requires extremely high peak currents, yet in conventional electron beams such compression generates a steady space-charge field that distorts the beam energy and quenches coherent amplification. Here we show, using three-dimensional particle-in-cell simulations and generalised FEL gain theory, that quasi-neutral electron--positron pair beams cancel this field at its source, preserving beam resonance without chirp compensation or slice selection. Simulations demonstrate that while an equivalent electron-only beam fails entirely, the pair beam produces ${\sim}1.85$-terawatt, $345$-attosecond pulses at $5.4\,\mathrm{nm}$ in a single ${\lesssim}\,6$-metre undulator, and ${\sim}10$-terawatt, $3.5$-attosecond spikes with coherent emission to $177\,\mathrm{keV}$ in a projected $18$-metre next-generation stage. These results establish a wavelength-universal operating mode with enhanced energy extraction providing a route to coherent gamma-ray emission for photonuclear spectroscopy and attosecond strong-field science.

[We respectfully request a consideration for the oral presentation in the Forum. This work (arXiv preprint is attached) resolves a fundamental and long-standing gain-suppression mechanism in ultrahigh-current FELs through a conceptually novel solution requiring no external hardware such as tapering: a quasi-neutral pair beam cancels the self-field detuning at its source. The result is a qualitative leap in accessible peak power and pulse duration via full-bunch lasing, with direct implications for the design of next-generation FEL facilities. The numbers obtained from the 3D-PIC simulations speak clearly: terawatt-level soft X-ray pulses and isolated 3.5 as spikes at 10 TW with coherent emission to 177 keV. Poster presentation of this work at the DESY Users' Meeting drew strong engagement from both FEL scientists and accelerator experimentalists. A given oral slot would allow the community to engage directly with the underlying physics and begin shaping the experimental roadmap this result demands. That is the reason then why we are much motivated for such an opportunity of oral contribution in the Forum.]

Speaker: Cagri Erciyes (Max Planck Institute for Nuclear Physics)

16:08 Light-driven Response of Collective Magnetic Excitations in an Electron-doped Cuprate via tr-RIXS

Unravelling the response of microscopic degrees of freedom in photoexcited cuprates is a central challenge in ultrafast materials science. While the charge response has been extensively investigated via time-resolved (tr-) optical spectroscopy and tr-ARPES, the nonequilibrium dynamics of magnetic degrees of freedom remain largely unexplored due to the lack of a direct probe. The advent of high-resolution RIXS instrument at high-repetition-rate X-ray FELs now enables high-quality tr-RIXS measurements at the Cu L-edge, providing a window into the underlying magnetic excitations. In this presentation, I will discuss our tr-RIXS experiments on the photoexcited electron-doped cuprate Nd2-xCexCuO4. We track the light-driven response of the paramagnon as a function of time and momentum, revealing a transient modification of the dispersion and a spectral weight redistribution within the Brillouin zone. These results highlight the intriguing prospects of manipulating magnetic degrees of freedom via light-matter interactions in quantum materials.

Speaker: Wei-Sheng Lee (SLAC National Accelerator Lab.)

16:09 Structure Recovery from Fluctuation X-Ray Scattering: A Rotational-Invariant Perspective

Fluctuation X-ray scattering (FXS) offers a complementary approach to imaging of nano- and bioparticles at X-ray free-electron lasers (XFELs) by extracting structural information from statistical moments of large ensembles of measured scattering patterns.
This poster reviews basic concepts behind FXS from the perspective of rotational invariants. Further more, we employed our software suite xFrame [1] to compute 3D virus reconstructions from several experimental datasets and present the results [2].

[1] T. B. Berberch et al., J. Appl. Cryst. 57, 324-343 (2024)
[2] T. B. Berberch et al., in submission

Speaker: Mr Ruslan Kurta (European XFEL)

16:10 Hard X-ray Free Electron Laser pulse mutiplexing and steering capabilities at the European XFEL

We present our pulse-multiplexing studies – one of the mid-term strategies at the European X-ray Free Electron Laser (European XFEL). The activity is motivated by hallmark XFEL properties—ultra-high intensity and a high degree of spatial coherence [1-2]—as well as a “MHz repetition rate” that can deliver approximately 400 pulses per each undulator-branch, at 220 ns intervals within a 10 Hz pulse-train format [1-2]. Since the launching of operation in 2018, a growing understanding is such that for various research fields, rarely the all three characteristics (high repetition rate, ultra-high intensity, coherence) are required, and typically the intensity or the repetition rates are excessive for the intended experiments; this then resulted to a significant number of XFEL pulses (or intensities) being discarded, whilst general accessibility to XFEL beam being extremely limited at the same time.
In this work, we address on practices that envision multiplexing mode at the SASE2 (hard X-ray) beamlines of the EuXFEL. Realisation of such scheme not only relaxes setup-time pressure significantly, but envisaged to enhance efficiency and experimental productivity at the same time. Methods that provide versatility and adaptability to changing operation modes are considered; via “redirection” of photon path with existing components; or via “multiplication” by introducing beamsplitting optics; and via manipulation of electron beam operation modes. Naturally, the aim also encompasses the requirement to preserve pulse integrities —such as the wavefront, intensity, and lateral coherence. Particular attention is given to optics which should withstand the extreme radiation and repetition rates. To this end, a preliminary analysis on the MHz repetition pulse wave front modulation is presented, where contributions originating from direct mechanical perturbations — such as low-frequency vibrations of X-ray distribution mirrors [4,6] — could be differentiated from indirect forces acting on electron bunches within the accelerator and undulator systems.
References
[1] Tschentscher, T et al., Appl. Sci. 7, 592. (2017)
[2] Decking, W. et al., Nat. Photon. 14, 391. (2020)
[3] Vagovic, P et al., Optica. 6, 1106 – 1109. (2019)
[4] Schmidtchen, S et al., Proc. SPIE 12694, doi: 10.1117/12.2677287
[5] Seaberg, M et al., J. Synch. Rad. 26, 1115 (2019)
[6] Guest, T.W. et al., J. Synch. Rad. 29, 939 (2022)

Speaker: Mikako Makita (European XFEL GmbH)

16:11 Circularly-polarized IR FEL pulses selectively probe dynamic processes and pump Rydberg states in doped semiconductors

Abstract:
Short, coherent pulses from infrared free electron lasers has been used extensively for studying population dynamics, and to a lesser extent coherence, of bound carrier states in semiconductors. Here we focus specifically on the hydrogen atom-like states of single donors and acceptors. Several challenges beset characterisation: the ladder of excited bound states is closely spaced in relation to the incident beam line width, and states lie very close to the conduction or valence band continuum. Measuring basic parameters such as the population life time $T_1$ is open to different interpretations. Coherent measurements such as photon echo and Ramsey spectroscopy offer a way to disambiguate this but require more complex setups than pump-probe. We demonstrate that by using a circularly polarized pump and a linearly polarized probe at ~10 THz we can harness the inherent optical dichroism of the excited Rydberg states to correctly attribute different decay paths of excited Rydberg states in phosphorus-doped silicon using only two beams, measuring optical Faraday rotations up to ±8°.

Access to circularly polarized IR FEL pulses allows us to harness spin selection rules to demonstrate optical pumping of hole spins in boron-doped silicon, analogous to spin preparation in atomic physics. We find a single dark state in the J=3/2 manifold of the hole’s orbital ground state that is not excited by circularly polarized light, and preferentially pump holes from the other Jz eigenstates into it. We recover two time scales: a fast (~30 psec) orbital decay and a slow (~1 nsec) spin decay. The measured decay and population transfer can be modelled with a rate equation and a few simple assumptions. We propose a realistic path to 99% spin initialization within hundreds of picoseconds, several orders of magnitude faster than current microwave initialization-based protocols.

Speaker: Guy Matmon (PSI - Paul Scherrer Institut)

16:12 Core-Level Chiral Response of gas-phase methyl oxirane Investigated by PECD and Ion Spectra

Chirality is a fundamental structural feature of many molecules and plays an important role in biological processes as well as in chemical synthesis via lock-and-key-type interactions. Although macroscopic methods exist to determine the enantiomeric excess of chiral molecules in chemical and biological environments, a full microscopic understanding of chirality-specific responses of individual molecules to external triggers, and of how these responses are modified by environmental influences, remains incomplete. Photoelectron circular dichroism (PECD) is a phenomenon driven purely by electric-dipole interactions and is therefore exceptionally sensitive to the chiral arrangement of atoms in a molecule. In contrast to other radiation-induced probes, which often rely on weaker magnetic-dipole or electric-quadrupole contributions, PECD exhibits a characteristic forward–backward asymmetry in the angular distribution of photoelectrons emitted upon ionization with circularly polarized light. This asymmetry reverses sign when either the light helicity or the molecular enantiomer is inverted.
In this work, we present static PECD measurements of gas-phase methyloxirane at the O 1s edge, where we observe a clear mirror-symmetric asymmetry. In addition, we report ion spectra recorded following O 1s ionization using EuXFEL pulses, together with complementary ion spectra measured in our laboratory at 800 nm. These data provide a basis for comparing core-level chiral response under different excitation conditions.

Speaker: Juliette Leroux (University of Hamburg)

16:00 → 20:00 Site Visit

Tuesday 22 September

08:30 → 10:30 AMO Physics

Convener: Gregor Knopp (PSI - Paul Scherrer Institut)

08:30 New advances in attosecond chemistry (Tutorial)

Speaker: Prof. Fernando Martin (IMDEA)

09:10 Probing Phase Variations Across the Argon 3s Amusia-Cooper Minima using Phase-Locked XUV Harmonics from a Seeded Free-Electron Laser (FERMI)

Abstract: The phase-locked extreme ultraviolet (XUV) harmonic comb generated by the UV-seeded free-electron laser (FEL) FERMI provides intense attosecond pulse trains (APTs) with controllable temporal waveforms [1]. The high degree of coherence, along with the tunability in wavelength and harmonic intensity of a seeded FEL, makes these APTs ideally suited for scanning across spectral features such as resonances and Cooper minima, enabling the extraction of atomic phases. In this work, we aim to measure the variation of the atomic phase across the argon (Ar) 3s Amusia-Cooper minimum (ACM) using the Reconstruction of Attosecond Beating by Interference of Two-photon Transitions (RABBITT) technique. We demonstrate the first implementation of a commensurate visible laser as a probe field in combination with the XUV APTs. To overcome timing jitter between the XUV and the probe pulses, we employ a timing tool based on single-shot correlation analysis of the sideband signal [2]. This approach allows us to retrieve the relative phase between the XUV harmonics and the visible probe pulse. By reordering the acquired spectra, we can get the phase variation of the Ar 3s channel relative to the 3p reference channel, observing the theoretically predicted phase dip in the vicinity of the 3s ACM [3]. Furthermore, we demonstrate that the controlled harmonics in the comb allow us to avoid contamination from shake-up processes. These results demonstrate the capability of seeded FEL-based RABBITT measurements to access subtle weak channel and multi-photon atomic phase variations and highlight their potential for time-resolved studies of electron dynamics in complex systems and future tests of causality on the attosecond time scale [4].
[1] P. K. Maroju et al., Nature, 386–391, 2020
[2] P. K. Maroju et al., Nat. Photon., 200–207, 2023
[3] M. Li et al., Phys. Rev. Lett., 183202, 2025
[4] R. Tahouri and J. M. Dahlström New J. Phys. 28 063205, 2026
Keywords: Attosecond; RABBITT; FEL; XUV.

Speaker: Arikta Saha (Elettra Sincrotrone Trieste)

09:25 Ultrafast X-ray pump X-ray probe spectroscopy at the Maloja endstation of SwissFEL

Development of new machine modes at large scale facilities like Free-Electron Lasers (FEL) opened new experimental possibilities to study X-ray induced dynamics. With the “fresh slice method”, two intense femtosecond X-ray pulses can be produced with independent and tunable photon energy and time delay. This beam mode can be used in pump-probe schemes, exploiting the high target specificity of X-rays combined with femtosecond time resolution to study dissociation dynamics for instance. Here, we demonstrate the capabilities of the Maloja endstation to perform ultrafast X-ray spectroscopic measurements with the 2-color mode developed for the soft X-ray Athos beamline at SwissFEL [1].
In the first presented experiment, X-ray Photoelectron Spectroscopy (XPS) was employed to study the core-excited ultrafast dynamics of CF4. The XPS differential map is presented in Figure 1 and shows, in addition to the CF4 ground-state bleach at 694eV, a very fast transient feature at 718eV that thereafter feeds into two different peaks CF3+ at 705eV and F+ at 727eV.
In a more recent beamtime, we performed X-ray Absorption Spectroscopy (XAS) of weakly bound water clusters to study the ultrafast relaxation pathways and competition of non-local processes like Intermolecular Coulombic Decay (ICD) and Proton Transfer (PT). We were able to disentangle an almost instantaneous signal, that we attribute to ICD, from a delayed channel corresponding to PT.
These experiments highlight the ability of Maloja to use an all-X-ray pump-probe scheme to investigate ultrafast processes with different targets (gas or molecular clusters) and observables (photoelectrons or fragmented ions).
References:
1- Prat, E. et al. Widely tunable two-color x-ray free-electron laser pulses. Phys Rev Res 4, (2022).

Speaker: Loïc Lucien Cédric Bassement (PSI - Paul Scherrer Institut)

pres.pdf

pres.pptx

09:40 Strong-field ionization from dynamically correlated excited states

The helium atom is the ideal prototype system to investigate the role of the initial state and its electronic correlation in the ionization process with high experimental and theoretical accuracy. Here, we present an XUV-pump-IR-probe coincidence spectroscopy study of strong-field ionization from doubly excited states (DES) in helium, with a particular focus on the role of dynamic electron correlation. The doubly excited states are selectively prepared using extreme ultraviolet radiation from the free-electron laser FLASH2 and are subsequently ionized in the presence of a delayed infrared field. Using the reaction microscope of beamline FL26 we measured three-dimensional photoelectron momentum distributions of electrons emitted from the $\text{sp}_{2,3+}$ and the $3\text{s}3\text{p}$ state during single ionization. The measured distributions, benchmarked against ab initio calculations [1] incorporating electron-electron interaction in two levels of complexity – static and dynamic correlation – show that both static and dynamic correlation contribute to the observed nodal structure of the resolved differential cross sections [2].
[1] L Argenti et al, Phys. Rev. Res. 5, 033047 (2023). [2] Borisova et al., In preparation

Speaker: Gergana D. Borisova (Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany; Department of Physics, Lund University, 221 00 Lund, Sweden;)

10:05 Quantum Control of an Atomic Transition through Rapid Adiabatic Passage with an XUV Free-Electron Laser

Free-electron lasers (FELs) are opening new regimes of light–matter interaction[1-3], and the advent of spectro-temporal shaping of intense XUV to X-ray pulses[4,5] is enabling quantum-control and non-perturbative photoexcitation in a previously inaccessible spectral range[6,7]. Rapid adiabatic passage (RAP) is a particularly robust quantum control protocol, providing efficient photoexcitation that is immune to intensity fluctuations and to spatial mode inhomogeneities of the driving field, yet it has so far been restricted to the UV-to-radiofrequency range. Here we demonstrate RAP quantum control of an atomic transition in the XUV, efficiently populating the narrow-linewidth, long-lived (~1 ns) 1s2p state of helium with an ultraintense, chirped seeded-FEL pulse. Experiments were carried out at the seeded FERMI FEL (Elettra, Trieste), with the photon energy tuned to the 1s²→1s2p transition at 21.18 eV and focused into a helium gas jet. The excited-state population was probed by linear ionization with a 1 ps delayed, 40 fs, 266 nm UV pulse and the resulting 1.3 eV photoelectrons were recorded in a velocity-map-imaging spectrometer, providing a direct measure of the 1s2p population. We compared a constant-frequency (~80 fs) resonant pulse against a chirped pulse (group-delay dispersion ~−1000 fs², ~110 fs) driving RAP, while scanning the FEL intensity over three orders of magnitude (~10¹¹–10¹⁴ W/cm²). Above ~10 µJ (peak intensity ~10¹³ W/cm²), RAP provides a significantly higher yield than the compressed pulse counterpart. Furthermore, theoretical models predict an excitation probability close to 1 for a large part of the spatial FEL mode.
These results open a new route to high-fidelity control of atomic ensembles in the XUV, with applications ranging from superradiance and correlation-driven phenomena[8], to the efficient population of narrow-linewidth nuclear transitions.

1. Shvyd’ko, Y. et al. Nature 622, 471–475 (2023).
2. Alexander, O. et al. Science Advances 10, eadp0841 (2024).
3. Driver, T. et al. Nature 632, 762–767 (2024).
4. Gauthier, D. et al. Physical Review Letters 115, 114801 (2015).
5. Rebernik Ribič, P. et al. Nature Photonics 13, 555–561 (2019).
6. Nandi, S. et al. Nature 608, 488–493 (2022).
7. Richter, F. et al. Nature 636, 337–341 (2024).
8. Pizzi, A. et al. Nature Physics 19, 551–561 (2023).

Speaker: Felix Ritzkowsky (DESY)

10:30 → 11:00 Coffee Break

11:00 → 13:00 Biology

Convener: Camila Bacellar (PSI - Paul Scherrer Institut)

11:00 Fighting quantum yield: two challenging SFX targets

Speaker: Thomas Lane (DESY)

11:40 Impact of Electron Trapping on Coulomb Explosion Imaging of Proteins at XFELs

Understanding the ultrafast response of proteins and large biomolecules to intense X-ray pulses is essential for advancing their structural and dynamical studies. A critical question for larger systems (>1000 atoms) is whether a complete Coulomb explosion can develop before electron trapping suppresses it. In this work, we examine the competitive dynamics between Coulomb explosion and electron trapping by probing the dynamics of the system in space and time using the XMDYN program. Our approach includes several processes such as: the photo-ionization, Auger-Meitner decay, collision-induced secondary ionization, and photofluorescence using the XATOM code. In this work we reveal whether a full Coulomb explosion suitable for imaging a protein can be achieved, or whether electron trapping rapidly suppresses it under realistic XFEL conditions available at the present.

Speaker: Kalyani Chordiya (University of Hamburg)

11:55 Crystal optics selects weak response of ultrafast crystallography

Time-resolved X-ray crystallography requires intense, short laser pulses to achieve ultrafast time resolution. It is currently not known to what extent double optical excitation as a result of intense laser illumination, can contribute to measured signals. We present a separation of weak and strong response from pump-probe data of photoactive yellow protein crystals collected at a single laser power, based on a method of indexing transformation followed by optical crystallography. We show that this allows precise separation of the heterogeneous photochemical populations with a very high level of data correlation that is intrinsic from the methodology of serial crystallography data collection. For a 3 ps pump-probe delay, the data shows evidence only for the trans-cis photoisomerisation reaction. Chromophore ionisation, which results from the double excitation in the strong response conditions, does not result in detectable crystallographic differences. The quantitative treatment of photoselection demonstrates that ultrafast time resolution can be established for single excitation dynamics of the functionally relevant photoisomerisation reaction of PYP [1].

1] Crystal optics select weak response of ultrafast crystallography. Perrett et al. ChemRxiv 2025 https://dx.doi.org/10.26434/chemrxiv-2025-72m69

Speaker: Jasper van Thor (Imperial College London)

12:20 Analysis of diffraction data from time-resolved experiments suggests incoherent addition of populations of states

Analysis of diffraction data from time-resolved experiments suggests
incoherent addition of populations of states

Current refinement of crystal structures with excited-state
populations commonly relies on extrapolated data sets (Schmidt et al.,
2003; De Zitter et al., 2022). These are derived from differences
between diffraction data collected from crystals containing mixed
ground- and excited-state populations and data from crystals in the
ground state, each measured independently.

Having atomic models for both ground and excited states available, we
sought to refine both structures simultaneously against diffraction
data from crystals with mixed populations. Instead of extrapolation,
we partitioned the observed data into contributions from each
structural state using coherent addition of structure
factors. Difference electron-density maps were calculated and
structures refined using an updated version of the MAIN software
(Turk, 2013).

Initial tests were performed using the PYP data set collected 3 ps
after laser excitation (Pande et al., 2016; PDB 5HDS). During these
analyses we observed that coherent addition of ground- and
excited-state contributions did not perform satisfactorily. Compared
with refinement against extrapolated data, refinement against
coherently split data yielded higher partial R-factors and visually
distorted electron-density maps, particularly in omit regions of
interest. We therefore explored an alternative formulation based on
incoherent addition, analogous to diffraction from merohedral twins.
Unexpectedly, this approach produced clearer difference
electron-density maps and lower R-factors. Moreover, refinement
converged toward larger excited-state populations (approximately 30%)
compared with the initially assumed 10%. Difference maps calculated
using observed intensities (Iobs) rather than amplitudes (Fobs), while
using phases from the dark structure, further improved map clarity,
largely independent of weighting schemes (I, I/σ, SVD, Q- or
K-weighting). In the next step, the analysis was extended to
ligand-bound crystal structures of SARS-CoV-2 Mpro with partial ligand
occupancy. In these cases as well, refinement based on incoherent
data splitting consistently outperformed the coherent formulation.
Because I-based metrics apply equally to coherent and incoherent
models, we propose that refinement and goodness-of-fit assessment in
such systems should preferentially be formulated in terms of
intensities rather than structure-factor amplitudes.

These results motivate a re-examination of how structural populations
combine in diffraction from mixed-state crystals from the time resolved, ligand binding to heavy atom isomorphues replacement data interpretation.

Acknowledgements: Henry Chapman, Oleksandr Yefanov, Chufeng Li are gratefully acknowledged for constructive discussions. Marius Schmidt is gratefully acknowledged for providing data from the PYP project.

References

De Zitter, E., Coquelle, N., Oeser, P., Barends, T.R.M., Colletier,
J.-P., 2022. Xtrapol8 enables automatic elucidation of low-occupancy
intermediate-states in crystallographic studies. Commun Biol 5,
640. https://doi.org/10.1038/s42003-022-03575-7

Pande, K., Hutchison, C.D.M., Groenhof, G., Aquila, A., Robinson,
J.S., Tenboer, J., et al. 2016. Femtosecond structural
dynamics drives the trans/cis isomerization in photoactive yellow
protein. Science 352, 725–729. https://doi.org/10.1126/science.aad5081

Turk, D., 2013. MAIN software for density averaging, model building,
structure refinement and validation. Acta Crystallogr D Biol
Crystallogr 69, 1342–1357. https://doi.org/10.1107/S0907444913008408

Schmidt, M., Rajagopal, S., Ren, Z., Moffat, K., 2003. Application of
Singular Value Decomposition to the Analysis of Time-Resolved
Macromolecular X-Ray Data. Biophysical Journal 84,
2112–2129. https://doi.org/10.1016/S0006-3495(03)75018-8

Speaker: Dusan Turk (Jozef Stefan Institute)

12:35 Ultrafast X-ray biological imaging without crystals

The quest to image single molecules played a crucial role in the development of X-ray free-electron lasers (XFELs). The ultra-bright, ultrafast pulses of XFELs enable the recording of a particle’s diffraction pattern before radiation damage destroys it—a concept first proposed by Neutze et al. (Nature, 2000) and experimentally demonstrated at FLASH over a decade ago (Chapman et al., Nature Physics, 2006). Since then, flash X-ray imaging (FXI) has been successfully applied to imaging live cells (van der Schot et al., Nature Communications, 2015), cellular organelles (Hantke et al., Nature Photonics, 2014), and viruses in both two (Seibert et al., Nature, 2011) and three dimensions (Ekeberg et al., PRL, 2015).
The advent of the European XFEL ushered in an era of high-intensity, high-repetition-rate, and high-data-rate XFELs, demonstrating that FXI could fully leverage these advancements. However, the ultimate goal of achieving sub-nanometer, high-resolution imaging remains unfulfilled.
In this talk, I will present the first three-dimensional X-ray image of single proteins in the gas phase. Additionally, I will discuss microsecond time-resolved measurements of protein dynamics in solution, obtained via SAXS/WAXS at the European XFEL. Finally, I will share preliminary results on pushing X-ray solution scattering to the limit of one particle per shot, FXI in solution, highlighting how ultrathin liquid sheets, coupled with advancements in X-ray optics, hold promise for ultrafast, nanoscale imaging of biological macromolecules in solution at room temperature.

Speaker: Filipe Maia (Uppsala University)

13:00 → 14:00 Lunch

14:00 → 16:00 Solid State Physics

Convener: Dr Simon Gerber (PSI - Paul Scherrer Institut)

14:00 Quantifying angular momentum of coherently driven circular phonons

Speaker: Roman Mankowsky (PSI - Paul Scherrer Institut)

14:25 Ultrafast terahertz manipulation of exciton complexes in 2D semiconductors

Coulomb-bound quasiparticles, such as excitons and trions in transition metal dichalcogenide monolayers, feature high binding energies on the order of 100s of meV, exhibit strong light-matter coupling, and can store quantum information in spin-valley degrees of freedom. Realizing strategies for external control of the excitonic states on ultrafast timescales thus emerged as an important avenue of research.

Here we discuss the observations of transient trion-to-exciton conversion in hBN-encapsulated MoSe$_2$ monolayer induced by intense THz pulses on picosecond timescales, generated at the infrared Free-Electron Laser facility (FELBE) [1]. The exciton dynamics are monitored by recording time-resolved photoluminescence (trPL) spectra with a streak camera which is synchronized at the repetition rate of the FEL. Visible pulses ($\lambda$ = 400 nm) excite a population of both excitons and trions. By adding THz pulses after about 30 picosecond delay with respect to the visible excitation, we observe a strong quenching of the trion emission and a temporary brightening of the exciton emission. Importantly, efficient trion-to-exciton conversion is observed when the THz photon energy is equal or higher than the trion binding energy.

Recently, we observed similar impact of THz radiation in other materials such as WSe$_2$ monolayers [2] and MoSe$_2$/WSe$_2$ heterostructures, that host more complex excitonic species such as charged biexcitons and interlayer excitons, respectively. Altogether, the results open promising pathways towards external control of many-particle states in low-dimensional materials.

References
[1] T. Venanzi, et al., Nat. Photonics, 18, 1344-1349 (2024)
[2] M. Cuccu, et al., Phys. Rev. B 112, 205302 (2025).

Speaker: Tommaso Venanzi (TU Dresden)

14:50 Terahertz saturable absorption from relativistic high-temperature thermodynamics in black phosphorus

Black Phosphorous (BP) represents a very unique two dimensional (2D) material due to its tunable infrared band gap and its anisotropic conduction properties. It has been emerged as an attractive material with a huge potential for future nanoelectronics and nanophotonic applications [1]. BP also shows the optical signatures of pressure induced Lifshitz transition from a semiconductor to a Dirac semi-metal due to emergence of a plasma of massless charge carriers [2]. Here we investigate the nonlinear terahertz (THz) electrodynamics of black phosphorus along the more conducting armchair direction. The THz saturable-absorption properties of black phosphorus can be understood within a thermodynamic model by assuming a fast thermalization of the electron bath which is similar to the case of other 2D systems such as graphene and topological insulators [3]. Our analysis shows that although BP does not show the presence of massless fermions at ambient pressure and temperature, its anomalous THz nonlinear properties can be accounted for by a relativistic massive Dirac dispersion, provided that the Fermi temperature is low enough. This suggests that an optimal tuning of the Fermi level could be a strategy to engineer a strong THz nonlinear response in other massive Dirac materials, such as transition-metal dichalcogenides or high-temperature superconductors.

References:
1. R. Fei et al., Topologically protected Dirac cones in compressed bulk black phosphorus, Phys. Rev. B 91 (2015) 195319.
2. P. Di Pietro et al., Emergent Dirac carriers across a pressure-induced Lifshitz transition in black phosphorus, Phys. Rev. B 98 (2018) 165111.
3. N. Adhlakha et al., Terahertz saturable absorption from relativistic high-temperature thermodynamics in black phosphorus, Phys. Rev. Appl. 20 (2023) 054039.

Speaker: Nidhi Adhlakha (Assistant Professor, National Institute of Technology Karnataka, Surathkal -575025)

15:05 Sum- Frequency Generation Spectro-Microscopy using the FHI-FEL

In this talk infrared-visible sum-frequency generation (SFG) spectro-microscopy using the FHI-FEL is introduced. This method combines benefits from second harmonic generation imaging such as local information on structural symmetry and benefits from sum-frequency generation spectroscopy such as details of the atomic structure and bonding via vibrational resonances.

We demonstrate an enhanced spatial resolution of a nineth of the IR-wavelength by imaging surface phonon polaritons in 4H-SiC nanostructures as a proof-of-concept [1]. In an extended work on a metasurface of SiC
micropillars, spectro-microscopy is shown to measure the polariton dispersion simultaneously in momentum space by angle-dependent resonance imaging, and in real space by polariton interferometry [2]. At last, we
demonstrate the feasibility of our method for spectroscopy using in-plane anisotropic wurtzite-type aluminum nitride as a model system [3] and show first data of our method’s application to ferroelectrics and 2D materials.

[1] R. Niemann, S. Wasserroth, G. Lu, S. Gewinner, M. de Pas, W. Schöllkopf, J.D.Caldwell, M.Wolf and A.Paarmann, “Long-wave infrared super-resolution wide-field microscopy using sum-frequency generation” Appl. Phys. Lett. 120(13),131102, (2022).
[2] R. Niemann, N. S. Mueller, S. Wasserroth, G. Lu, M. Wolf, J. D.Caldwell and A. Paarmann,“Spectroscopic and Interferometric Sum-Frequency Imaging of Strongly Coupled Phonon Polaritons in SiC Metasurfaces” Adv. Mat. 36(33),2312507, (2024).
[3] D. S. Mader, R. Niemann, M. Wolf, S. F. Maehrlein and A.Paarmann,” Sum-frequency generation spectro-microscopy in the reststrahlen band of wurtzite-type aluminum nitride” J. Chem. Phys. 161, 094706 (2024).

Speaker: Dorothee Mader (Fritz-Haber-Institut der Max-Planck-Gesellschaft)

15:20 EUV diffuse scattering spectroscopy of nanoscale surface phonon anisotropy in optically excited FeRh

Nanoscale ultrafast all-optical tuning of magneto-acoustic devices is of significant scientific and technological interest, particularly in areas of energy efficient data storage and information processing. Ultrafast generation and control of high frequency nanoscale surface acoustic waves (SAWs) in the first order magneto structural phase transitions can be used to precisely control such devices. Utilizing recently developed time-domain Extreme ultraviolet diffuse scattering spectroscopy, we demonstrate the generation and detection of broadband nanoscale surface phonons featuring high frequencies and large wavevectors, exhibiting anisotropic propagation in biaxially anisotropic epitaxial BCC FeRh thin films. We probe the surface phonons in layered stack of Pt(3 nm)/FeRh (40 nm) /3C-SiC (100 nm)/Si (381 $\mu$m) within the in-plane wavevector range of 0.028 nm$^{-1}$- 0.1 nm$^{-1}$ and frequency range of upto 50 GHz corresponding to phonon wavelengths of 62 nm -224 nm. We observe the square-shaped contours in time-domain scattering and frequency-domain iso-frequency curves corresponding to bi-axial anisotropy in cubic epitaxial FeRh which agree with corresponding theoretical simulations.

Speaker: Naman Agarwal (FERMI FEL, Elettra Sincrotrone Trieste, Trieste, Italy)

15:35 Pilot tr-XPS experiments at European XFEL: From non-equilibrium charge carriers in InAs to non-equilibrium charge-density waves in TaSe₂

We report pilot time-resolved X-ray photoemission spectroscopy (tr-XPS) experiments performed at the SXP (Soft X-ray Photoemission) instrument [1] of the European XFEL. The samples were pumped with 1.5 eV laser pulses at fluences up to 2 mJ/cm⁻² and probed with XFEL pulses of 1 keV with a fluence of ~5 μJ/cm⁻², a temporal resolution of ~60 fs and an energy resolution of ~100 meV. We explored non-equilibrium physics of two systems:
Non-equilibrium charge carriers in InAs. In equilibrium, the downward band bending at the InAs(110) surface creates a built-in electric field. Following pump excitation, this field drives photoexcited electrons toward the surface and holes into the bulk, resulting in the surface photovoltaic (SPV) effect. However, because the electron mobility exceeds that of holes by about two orders of magnitude, electrons diffuse much more rapidly into the bulk than holes, lagging near the surface. The resulting transient charge separation (the photo-Dember effect) forms a dipole layer that counteracts the SPV-related drift of the charge carriers [3]. We follow the transient interplay between carrier drift and diffusion through the time-dependent energy shifts of the In 4d core levels measured with the probe X-ray pulse.
Non-equilibrium charge-density waves (CDWs) in TaSe2. The non-equivalent Ta sites in the commensurate-CDW phase of TaSe2 show up in the XPS spectra as energy splitting of the Ta 4f core levels [2]. The pump excitation transiently suppresses the CDW order, which is reflected in the reduction of the Ta 4f splitting magnitude. The remnant splitting may reflect either persistence of the equilibrium-CDW domains, a reduction in the amplitude and/or long-range phase coherence of this phase, or the transient formation of high-temperature CDW phases. The equilibrium-CDW state recovers on a time scale of ~20 ps. This transient process overlaps with periodic oscillations of the Ta 4f splitting, which reflect coherent excitation of CDW modes coupled to dipole-active lattice vibrations.
These two cases illustrate the unfolding power of tr-XPS for exploring dynamics of non-equilibrium electronic processes in various solid-state systems.
[1] P. Grychtol et al. The diagnostics and laser infrastructure at the Soft X-ray Port of the European XFEL. J. Synchrotron Radiat. 33 (2026) in print
[2] A. Reklaitis. Terahertz emission from InAs induced by photo-Dember effect: Hydrodynamic analysis and Monte Carlo simulations. J. Appl. Phys. 108 (2010) 053102
[3] S. Hellmann et al. Ultrafast Melting of a Charge-Density Wave in the Mott Insulator 1T-TaS₂. Phys. Rev. Lett. 105 (2010) 187401

Speaker: V. N. Strokov (Center of Photon Science, Paul Schererr Institute, Villigen-PSI, Switzerland)

16:00 → 16:30 Coffee Break

16:30 → 18:10 Solid State Physics

16:30 Ultraconfined Terahertz Phonon Polaritons in Hafnium Dichalcogenides

Phonon polaritons are hybrid light-matter quasiparticles in polar crystals that enable waveguiding of light on length scales much smaller than the photon wavelength [1]. Here, we introduce the hafnium dichalcogenides HfS2 and HfSe2 as new van der Waals materials that supports phonon polaritons in the terahertz (THz) spectral range. Using THz near-field optical microscopy coupled to the infrared free-electron laser facility FELBE [2] at the Helmholtz-Zentrum Dresden-Rossendorf, we demonstrate extreme confinement of light from 61 𝜇m free-space wavelength to 245 nm [3]. We show that the origin of this record-high confinement is an exceptionally large light-matter coupling of hyperbolic HfSe2. We also demonstrate hyperlensing as a means to verify the hyperbolicity of HfSe2. Our work opens up new avenues to deeply subdiffractional THz technologies [4].

[1] Galiffi et al., Nature Reviews Materials 9, 9 (2023)
[2] Helm et al., Eur. Phys. J. Plus 138 (2), 158 (2023)
[3] Kowalski et al., Nature Materials 24, 1735 (2025)
[4] Paarmann & Caldwell, Nature Materials 24, 1688 (2025)

Speaker: Alexander Paarmann (Fritz-Haber-Institut der Max-Planck-Gesellschaft)

16:55 Imaging the non-equilibrium dynamics of resonantly excited metal nanorods

The melting of metallic nanoparticles under optical excitation has been one of the puzzling processes, wherein photoexcited electrons deposit their energy into the lattice via electron-phonon coupling on sub-picosecond time scales and consequent phonon dynamics takes over. While previous attempts [1,2] on gold nanoparticles have elucidated the electronic dynamics and the shape transformations, a complete dynamical trajectory of the system remains unresolved. This demands an extensive statistical analysis over large datasets.
We address this challenge by exploiting the single-particle-imaging technique at the European XFEL in a time-resolved pump-probe (pp) experiment on gold nanorods. More than 1M diffraction frames with precise pp-delay encoding were collected with a hit rate approaching 50%. With a plasmon resonant optical excitation (770nm) and pump fluences (3-8 mJ/cm2) much lower than the previous experiments [2,3], we suppress explosive fragmentation while preserving high data quality.
Within the first 100ps, a radial coherent phonon mode (period ~10ps) is observed which attributes to an acoustic phonon mode of the gold lattice, which subsequently decoheres and anharmonically couples to other vibrational modes within 60ps. Post melting, we observe an oscillatory rod-reshaping between ellipsoids and spheres, attributed to the elastic properties of the material. Hence, we attempt to comprehend for the first time a full dynamical trajectory in the first nanosecond after light absorption without ensemble averaging.

References:
1. Rubio et al., Acc. Chem. Res. 49 (2016)
2. Park et al., Nat Commun. 16 (2025)
3. Plech et al., ACS Nano 18 (2024)

Speaker: Dr Omkar Rambadey (Max Planck Institute for the Structure and Dynamics of Matter)

17:10 Structural Origin of the Magnetic Field-Induced Electric Polarization of GdMn$_2$O$_5$

We present the status and capabilities of the pulsed magnetic field setup that is currently being comissioned at the Cristallina-Quantum experimental station of the Aramis hard X-ray beamline branch at SwissFEL. The setup is based on a split-pair coil magnet, providing a vertical magnetic field with wide horizontal scattering angle access and compatible with cryogenic sample environments. The ultimate design goals are reaching magnetic fields of up to 40 T and low temperatures down to 1 K. The magnetic field pulses are synchronized to the femtosecond X-ray pulses from SwissFEL, enabling X-ray diffraction under simultaneous high magnetic field and low temperature conditions. The experimental scope is focused on high-field investigations of quantum materials, such as resolving the structural evolution of ordered state through field-driven transitions.

As a demonstration of this capability, we present the results from a study of the multiferroic GdMn$_2$O$_5$, where electric polarization and magnetic order are strongly coupled [1]. The compound exhibits a magnetic field-induced electric polarization with a component along the crystallographic a-axis that persists even after field removal, suggesting a path-dependent ground state [2]. The microscopic mechanism behind this persistent polarization remains unclear and resolving it requires a direct structural probe to follow the lattice response in the presence of high magnetic fields. Using X-ray diffraction under pulsed magnetic fields, we directly track the structural response of the lattice across this field-driven transition. Our measurements are guided by density functional theory simulations covering scenarios ranging from simple atomic displacements of the magnetic Gd ions to lattice relaxation driven by changes of the magnetic order. These models predict intensity variations of specific lattice Bragg peaks, providing a direct experimental benchmark for the structural origin of the magnetic field-induced electric polarization.

[1] G. Yahia et al. Recognition of exchange striction as the origin of magnetoelectric coupling in multiferroics. Phys. Rev. B 95, 184112 (2017).

[2] V. Balédent et al. Electronic ground-state hysteresis under magnetic field in GdMn${_2}$O${_5}$. Phys. Rev. B 108, 104419 (2023).

Speaker: Karina Kazarian (PSI - Paul Scherrer Institut)

Science@FELs_GdMn2O5.pdf

Science@FELs_GdMn2O5.pptx

17:35 Ultrafast Coherent Diffraction Imaging and Bragg CDI on individual Gold Nanoparticles

Ultrafast and intense X-ray free electron laser (XFEL) pulses can visualize non-equilibrium dynamics on the femtosecond time scale with high spatial resolution at single nanoparticle (NP) level . Single exposure coherent diffraction imaging (CDI) of individual nanospecimen has been so far limited to a few nanometers. The limiting factor is sample bleaching through photo-ionization and resulting low photon signal in higher scattering angles.

Our preliminary data analysis from experiments at LCLS and the EuXFEL indicates, that transient resonances might overcome bleaching in images recorded with intense, hard X-rays pulses and increase the spatial resolution. We have recorded small and wide-angle scattering (Bragg CDI) from Au NPs with sizes ranging from 4-70 nm. We observe multiple Bragg peaks from a single domain 10 nm NP, which exhibit multiple higher diffraction orders. The small angle X-ray diffraction signal reaches up to 1 nm resolution in images of samples, where the best previously reported reconstruction achieves only 2 nm resolution. Our data also suggests that 10 fs FEL pulses might be too long to observe true atomic resolution in single metal particle imaging.

By combining small angle CDI and wide angle BCDI, we obtain complementary structural information, with both techniques capturing multiple diffraction orders. This proof-of-concept study highlights a pathway for improving single-shot resolution and offers new insights into ultrafast structural dynamics at the nanoscale.

Speaker: Rasmus Buchin (CFEL/UHH)

17:50 EUV Diffuse Scattering for Nanoscale Acoustic Phonons and Thermoelasticity

Thermoelastic properties at the nanoscale play a central role in condensed matter physics and nanostructured materials, yet remain challenging to probe experimentally. Extreme ultraviolet (EUV) transient grating (TG) technique has recently enabled access to these dynamics at selected wavevectors in the tens-of-nanometers range, providing insight into nanoscale thermal transport and acoustic phonons [1]. More recently, a novel time-resolved technique based on diffuse EUV scattering (DES) has emerged as a simple and effective method for studying phonons with nanoscale wavelengths [2]. In DES, an ultrafast optical pump excites lattice dynamics, while a coherent EUV probe measures the resulting diffuse scattering pattern. The scattered intensity forms a time-dependent fringe pattern on a two-dimensional detector, where each point corresponds to a specific wavevector q and oscillates at the frequency of the corresponding optically excited phonon. DES thus provides direct access to the dynamics of acoustic phonons spanning wavelengths from ~60 to 400 nm. Compared to EUV TG, DES offers several key advantages, including continuous q-access within a single measurement, linear response to excitation fluence, and signal enhancement arising from surface roughness.
In this talk, we will introduce the DES technique and present its application in transmission geometry, which enables access not only to surface modes but also to bulk acoustic waves propagating in thin films. In addition to mapping the dispersion of high-wavevector guided modes, the observation of resonances associated with zero-wavevector longitudinal modes provides insight into the excitation mechanism, indicating a dominant role of thermoelastic generation followed by scattering from surface roughness.
Looking ahead, this approach can be naturally extended to the X-ray regime, opening the way to probing dynamics at even shorter length scales. DES also offers new opportunities to investigate magneto-elastic coupling, by enabling access to high-wavevector phonons and their interaction with magnetic degrees of freedom.

[1] Foglia et al., Photoacoustics 29, 100453 (2023)
[2] Capotondi et al., Phys. Rev. Lett. 135, 266101 (2025)

Speaker: Alessandra Milloch (ELLETRA Sincrotrone Trieste)

18:35 → 21:05 Dinner

Wednesday 23 September

08:30 → 10:30 Biology

Convener: John Henry Beale (PSI - Paul Scherrer Institut)

08:30 Integrated time-resolved structural biology approaches to visualize photoreceptor proteins at work

Photoreceptor proteins utilise the absorption of light to elicit a biological response in a variety of processes, including vision, circadian rhythms and plant development. The focus of current research is the understanding of how the initial photochemical events at the receptor's chromophore funnel through space and time into the desired photobiological event.
In order to visualise photoreceptor proteins in action, a particularly powerful approach is to integrate results from various time-resolved structural biology approaches. Time-resolved serial femtosecond crystallography at XFELs provides atomic resolution structural snapshots of photo-intermediates at physiological (room) temperatures. A potential disadvantage of this approach is the possible interference of structural changes with crystal packing. Time-resolved X-ray solution scattering overcomes this limitation, provides kinetic information on photo-intermediates but the structural information is of low spatial resolution. Time-resolved single particle cryo-EM also provides an atomic-resolution view of structural changes not limited by crystal packing but involves data collection at non-physiological (cryo) temperatures. The merits of a synergistic implementation of time-resolved structural biology approaches will be exemplified by a recent study that investigates a vitamin B12-dependent photoreceptor.

Speaker: Martin Weik (IBS)

09:10 Sub-picosecond Low-Barrier Hydrogen Bond Collapse Preceding Proton Transfer Reaction in a Light-Driven Ion Pump

Abstract:
Proton transfer underlies enzymatic catalysis, ion transport, and energy conversion across biology, often involving short, strong H-bonds in which the proton is delocalized between two functional groups. Such low-barrier H-bonds (LBHBs) have been shown in protein active sites, but their dynamics are difficult to capture in this environment because LBHBs unfold on ultrafast timescales and with minor atomic rearrangement - the combination which is inaccessible to most structural methods. In my talk I present our recent high-resolution time-resolved serial femtosecond crystallography (TR-SFX) data collected at the Alvra station of SwissFEL on the light-driven Na+ pump ErNaR containing a functionally-relevant LBHB between two carboxylic residues and in direct interaction with its retinal Schiff base. This LBHB is identified by a characteristic infrared continuum band and is confirmed by proton and deuterium delocalization in quantum mechanics simulations. TR-SFX show the LBHB collapse in 600 ps of photon absorption by retinal, followed by microsecond-to-millisecond time scale rearrangements leading to the Schiff base deprotonation. I will also discuss how an ultrafast electronic event in the retinal cofactor is linked to a biological function occurring orders of magnitude later.

Speaker: Kirill Kovalev (European Molecular Biology Laboratory)

09:35 Sample delivery for structural biology at the European XFEL

The European XFEL (EuXFEL) facilitates high-resolution structural studies of biological systems at MHz pulse repetition rates. Serial femtosecond crystallography (SFX) - next to scattering and imaging techniques - is the most common approach. Time-resolved structural studies are enabled with optical pump-probe or mixing experiments. To accommodate the MHz repetition rate, sample delivery methods are in demand which ensure quick and efficient sample renewal. The Sample Environment and Characterization (SEC) group at EuXFEL develops these methods and supports users during their experiments. In this presentation developments in sample delivery via liquid jet injection, high viscosity extruder, drop-on-demand and fixed targets will be discussed. Additionally, the recently introduced beamtime access via the QUick Access Screening Time (QUAST) approach and the SEC user support will be described.

Speaker: Katerina Dörner (European XFEL)

09:50 Sulfur Photochemistry in Human γD-Crystallin

The eye lens has evolved to retain transparency throughout the organismal lifespan in the absence of cellular regeneration. The crystallin proteins that constitute the lens remain folded and soluble for more than 50 years in humans despite daily exposure to ultraviolet (UV) radiation. Human γD-crystallin (HGD) is one of the principal structural proteins of the lens. Long regarded as a passive structural protein, growing evidence indicates that HGD participates actively in light-driven redox chemistry. HGD provides a unique system for investigating the molecular mechanisms that protect long-lived proteins from photodamage (1). Oxidative ageing drives the formation of intramolecular disulfide bonds and glutathionylated cysteine residues through sulfur-centred chemistry. We hypothesise that these transient modifications enhance quenching of excited tryptophan residues to suppress further photooxidation, limiting the formation of intermolecular disulfide bonds that ultimately promote protein aggregation and cataract formation (2).

This work combines time-resolved X-ray crystallography with complementary X-ray spectroscopic techniques (3) to investigate sulfur photochemistry across timescales ranging from femtoseconds to milliseconds. XFEL measurements have established a mechanistic framework for understanding how HGD mitigates oxidative stress through controlled redox transitions. Capturing transient sulfur-centred intermediates, these experiments allow structural changes to be correlated with the spectroscopic signatures of evolving electronic states. Complementary femtosecond X-ray absorption spectroscopy enables us to identify the timescales and chemical identities of sulfur radical and ionic species formed during disulfide photochemistry, providing direct insight into the earliest events following photoexcitation. Together, these approaches define light-induced redox pathways by linking ultrafast electronic processes with protein structural dynamics. This integrated view of sulfur photochemistry provides new insight into the molecular mechanisms of photoinduced oxidative ageing in the lens and eventual cataract formation.

(1) Schafheimer, N. and King, J. (2013), Tryptophan Cluster Protects Human γD-Crystallin from Ultraviolet Radiation-Induced Photoaggregation In Vitro. Photochem Photobiol, 89, 1106-1115.
(2) Hill, J.A., Nyathi, Y., Horrell, S., von Stetten, D., Axford, D., Owen, R.L., Beddard, G.S., Pearson, A.R., Ginn, H.M. and Yorke, B.A. (2024), An ultraviolet-driven rescue pathway for oxidative stress to eye lens protein human gamma-D crystallin. Communications Chemistry, 7(1), p.81.
(3) Ochmann, M., Harich, J., Ma, R., Freibert, A., Kim, Y., Gopannagari, M., Hong, D.H., Nam, D., Kim, S., Kim, M., Eom, I., Lee, J.K., Yorke, B.A., Kim, T.K. and Huse, N. (2024). UV photochemistry of the L-cystine disulfide bridge in aqueous solution investigated by femtosecond X-ray absorption spectroscopy. Nature Communications, 15(1), p.8838.

Speaker: Briony Yorke (School of Chemistry)

10:15 Time-resolved GPCR photopharmacology at next-generation X-ray sources

Jörg Standfuss

representing collaborators at PSI Center for Life Sciences, PSI Center for Photon Science, LeadXpro AG, Institute for Advanced Chemistry of Catalonia, Stanford Cancer Institute, SLAC National Accelerator Laboratory, and University of Zurich

Abstract:
The advent of next-generation synchrotrons and X-ray free-electron lasers enables time-resolved structural biology at atomic resolution and femtosecond time resolution. At the Swiss Light Source (SLS), the Swiss X-ray Free Electron Laser (SwissFEL), and the Linac Coherent Light Source (LCLS), we use photoactive azobenzene compounds to trigger and follow drug–protein interaction dynamics in real time.
I will present time-resolved studies of the light-switchable drug candidate azo-combretastatin A4, tracking the process from the initial photochemical event on the femtosecond timescale to ligand release from tubulin on the millisecond timescale. Extending this approach to G protein–coupled receptors (GPCRs), we used photoswitchable compounds to follow ligand dissociation from the adenosine A2A receptor and the metabotropic glutamate receptor 5 (mGlu5). We further show how a photopharmacological compound can act as an efficacy switch, modulating β2-adrenergic receptor activity from within the binding pocket. These time-resolved structural snapshots provide new input for computational studies and for understanding protein–ligand interaction dynamics in important drug targets.

Speaker: Jörg Standfuss (PSI - Paul Scherrer Institut)

10:30 → 11:00 Coffee Break

11:00 → 13:00 Chemistry

Convener: Camila Bacellar (PSI - Paul Scherrer Institut)

11:00 IR characterization of CO2 adsorption on metal clusters: model systems for the catalytic active site

Gas-phase metal clusters can act as idealized model systems for the active sites of more complex catalysts, allowing for studies aimed at a fundamental understanding of metal-molecule interactions. In these experiments, we focus on understanding the adsorption of simple molecules on the model catalysts and establish the influence of local geometric and electronic structure as well as elemental composition. We do these investigations using a combination of mass-spectrometry, allowing studies of clusters with precise atomic and el-emental composition, and IR spectroscopy using the FELIX free-electron laser suite. The ex-perimental work is complemented by DFT calculations, allowing for structure determination and a rationalization of the formed products.
In this contribution, we present our results on the adsorption of CO2 onto metal clusters, and show how the variation of charge state (cations, anions), and elemental composition (type of metal, single element, multi-element) affect the cluster’s chemical activity.

Speaker: Joost Bakker (HFML-FELIX & Radboud University)

11:25 Does Rotation Matter? Mapping Charge Transfer In Dissociating Molecules

Achieving a molecular-level understanding of complex chemical reactions requires a detailed mapping of their elementary dynamics. Among these, charge transfer during two-body molecular dissociation has been investigated in pioneering experiments as a function of the distance between the two separating fragments (see e.g., Erk et al. 2014 and Schnorr et al. 2014). Here we build upon a study by Köckert et al. (2022), investigating how fragment orientation influences charge transfer during photo-dissociation. We employ an ultra-short pump pulse to trigger two-body dissociation in bromoiodomethane and chloroiodomethane. Because the halomethyl fragment’s center of mass is off-axis relative to the cleaved C–I bond, dissociation induces a rapid rotation of the neutral fragment. Subsequent to triggering the photo-dissociation, at well-controlled, varied time delays, we site-selectively ionize the iodine atom using an intense X-ray pulse. The occurrence of charge transfer following this ionization, as deduced from the detected ionic products and their kinetic energies, allows us to map the probability for charge transfer depending on charge state configuration and fragment orientation. Signals are isolated by mass, charge, kinetic energy, and delay using a velocity map imaging spectrometer combined with a Timepix3 nanosecond-timestamping camera. In this work, we present the successful extraction of these rotational signatures from our multi-coincidence data. We map the charge transfer probability across a wide range of charge states as a function of both distance and orientation, providing direct evidence of the system crossing the classical over-the-barrier critical distance, at which charge transfer is inhibited, multiple times due to fragment rotation.

Speaker: Björn Senfftleben (ETH Zürich)

11:40 Terahertz driven nanosecond dynamics of oxygen defect state in anatase TiO2−x

Ultrafast nonlinear terahertz (THz) spectroscopy is applied at the TeraFERMI beamline of the Italian FERMI free electron laser (FEL), where intense (multi MV/cm peak field), broadband (up to 4 THz) and ultrashort (< ps) THz pulses are generated by coherent transition radiation from the relativistic electron bunches of the FEL [1].
The beamline allows addressing the THz nonlinear properties of materials, ranging from quantum materials to bio-chemical samples, by measuring the single(two)-color THz pump-THz(infrared) probe response.
Here, we show an example on titanium dioxide, a largely available and naturally occurring functional oxide, and a benchmark among photocatalytic materials.
Oxygen vacancies in titanium dioxide have a strong impact on its photoactivity, by introducing band-gap states and consequently extending the photoresponse from the UV to the visible light region [2].
The time-resolved THz pump/infrared probe measurements allow achieving control on the oxygen defect state in TiO2-x, by modulating the infrared response associated to photoexcitation of oxygen vacancies under the intense terahertz short pulses delivered by TeraFERMI. The experiment explores the polaron dynamics around the oxygen vacancies, by unveiling the conversion of deeply localized defect states into long-lived metastable states, with a lifetime in the nanosecond range [3].
This approach is completely new, being based on a pump-probe experiment on titanium dioxide where a sub-gap terahertz pump beam is used for the first time. Our new finding impacts the exploitation of oxygen deficient titanium dioxide, a functional material largely usable for solar energy conversion, as well as in novel opto-electronic applications.

[1] P. Di Pietro et al., Synchrotron Radiation News 30, 4, 36 (2017).
[2] P. Orgiani et al., Phys. Rev. Applied 13, 044011 (2020).
[3] P. Di Pietro et al., Phys. Rev. Research 7, 023011 (2025).

Speaker: Paola Di Pietro (Elettra-Sincrotrone Trieste)

11:55 From Time-Resolved X-ray Solution Scattering to Femtosecond X-ray Spectroscopy at the FXL Endstation of PAL-XFEL

The Femtosecond X-ray Liquidography (FXL) endstation at PAL-XFEL is dedicated to femtosecond X-ray studies of solution-phase and condensed-phase samples. It combines femtosecond XFEL pulses with synchronized optical laser excitation and supports time-resolved X-ray scattering, X-ray absorption spectroscopy, X-ray emission spectroscopy, and time-resolved crystallography over an X-ray energy range of 2.3–20 keV. These methods provide complementary information on transient molecular structures and element-specific electronic and local structures in ultrafast chemical dynamics.

FXL was originally designed to support both time-resolved X-ray solution scattering and X-ray absorption spectroscopy. Both experimental directions have evolved through user operation and beamline developments. For scattering and diffraction experiments, FXL has been used for user studies of solution-phase photochemical reactions and TR-SAXS studies of deeply supercooled water and ice, and is being extended toward fixed-target and time-resolved crystallography approaches for organic and inorganic crystalline samples. These examples demonstrate the practical use of FXL for time-resolved scattering and diffraction experiments on liquid, condensed-phase, and microcrystalline samples.

For spectroscopic experiments, FXL has been expanded from XAS toward a broader range of fluorescence-based and high-resolution measurements. Self-seeded femtosecond X-ray absorption spectroscopy has been demonstrated for time-resolved XANES measurements, and tender X-ray absorption at edges such as the S K-edge and Ru L$_3$-edge is being explored. X-ray emission spectroscopy and high-resolution fluorescence-detected measurements based on von Hamos and Johann-type spectrometers are also being developed.

In this poster, we summarize the present status of the FXL endstation and representative user and beamline experiments performed at PAL-XFEL. The poster focuses on the current capabilities of FXL and its expansion toward a broader femtosecond X-ray scattering, spectroscopy, and diffraction platform for ultrafast chemical science.

Speaker: Jae Hyuk Lee (Pohang Accelerator Laboratory)

12:10 Ultrafast photoinduced electronic and structural modifications in CeO2

Metal oxides exhibit remarkable catalytic properties that make them promising candidates for efficient photoelectrochemical applications. Carrier lifetime and mobility play a key role in determining their catalytic performance, making it crucial to investigate the ultrafast electronic and structural dynamics induced by photoexcitation. In CeO₂, functionality relies on the reversible exchange of oxygen ions with reactants, and the formation of photoinduced polarons can enhance reducibility and photocatalytic activity. The associated structural distortions may further lower the oxygen vacancy formation energy, improving redox functionality. A previous pump-probe optical spectroscopy study revealed an ultrafast blue shift in photoinduced absorption by 0.4 eV, attributed to the formation of small polarons within 330 fs from the excitation [1]. The present study [2] examines the dynamic evolution of the electronic and crystal structure in stoichiometric CeO₂ films following photoexcitation, employing free-electron laser based pump-probe X-ray absorption spectroscopy at the Ce L$_3$ edge in both the near-edge (XANES) and extended energy (EXAFS) ranges. The analysis of the differential pump probe spectra in the XANES region and of the dynamic evolution of the signal intensity revealed that in the first few tens of fs the electronic structure undergoes modifications that are compatible with a transient occupation of Ce 4f states. Within ~ 500 fs, the electronic structure partially relaxes to a metastable photoexcited state. This partial relaxation is accompanied by a simultaneous structural expansion of about 0.1 Å in the first shell of O atoms around the excited Ce ion as determined by the analysis of pump-probe spectra in the EXAFS region, reinforcing the hypothesis of formation of a photoinduced polaronic state. The electronic and structural modifications persist for more than 300 ps [2].

References:
1. J. S. Pelli Cresi et al., J. Phys. Chem. Lett. 11, 5686 (2020).
2. S.Pelatti, E.Spurio, D.Catone, et al. Adv. Electron. Mater.1 (2025)

Speaker: Eleonora Spurio (CNR)

EleonoraSpurio_scienceFELs.pptx

12:25 Femtosecond soft X-ray absorption spectroscopy on liquids at the Swiss Free Electron Laser

The K edges of light elements reveal the crucial role of the ligands in mediating ultrafast decay cascades. In this talk, I will present our investigation of a prototypical spin crossover complex, iron II trisbipyridine. Upon optical excitation, the ensemble of molecules undergoes a cascade of relaxation processes starting from the singlet states, intersystem crossing to the triplet state, and finally relaxing to the quintet state, all within the first 400 fs [1,2,3]. The transition to the final quintet state is accompanied by a pronounced elongation of the Fe-N bond [1,2]. By probing the nitrogen 1s to π* resonance, we isolate the specific changes in the local environment of the coordinating nitrogen sites during this ultrafast cascade. In the figure, we present transient absorption spectra that reveal a prompt shift and delayed rise in the nitrogen absorption resonance, which is a signature of the intersystem crossing.

a) Transient absorption spectra over the first 0.8 ps; b) Experimental scheme used for self-referencing transmission measurements.

Beyond this specific case study, I will present the experimental setup available at the Maloja endstation and offered to users. TR-XAS in the transmission geometry for liquids is available in the energy range of 260 eV to 1000 eV, coupled with a wide variety of optical wavelengths. The liquid injector is based on the micro-chip design by Koralek et al [4], allowing tunable sheet thickness from several 10s of nanometers to microns.
References:
1- K. S. Kjær, Chemical Science 2019, 10, 5749.
2- B. E. Van Kuiken, The Journal of Physical Chemistry Letters 2016, 7, 465.
3- G. Auböck, M. Chergui, Nature Chemistry 2015, 7, 629.
4- Koralek et al. Nature Communications volume 9, 1353 (2018)

Speaker: Andre Al Haddad (PSI - Paul Scherrer Institut)

13:00 → 14:00 Lunch

14:00 → 16:00 X-ray coherent imaging

Convener: Christoph Bostedt (PSI - Paul Scherrer Institut)

14:00 From X-ray damage reversal to single-exposure 3D nanoscale imaging via intense sub-femtosecond pulses

Coherent diffractive imaging of individual nanoparticles at FELs faces fundamental constraints due to rapid, dose-induced sample bleaching. This talk presents the first X-ray diffraction imaging experiments on individual nanoparticles utilizing intense, sub-femtosecond (sub-fs) pulses. At extreme peak power densities up to 10^20 W/cm², our data indicates that sub-fs exposures unlock novel regimes for manipulating the X-ray refractive index. Our data suggests that sub-fs pulses effectively mitigate and reversing transient radiation damage, and improve the quality of diffraction images. Furthermore, we demonstrate that the fluorescence emission excited during this ultra-short window encodes 3D structural data, accessible within a single exposure.
These findings demonstrate the untaped potential of sub-fs X-ray pulses for high-resolution, element-specific diffraction imaging. This approach opens a viable pathway to capture transient, non-equilibrium states of matter and resolve structural dynamics in systems like active catalytic reactions in real time.

Kuschel, Stephan, et al. "Non-linear enhancement of ultrafast X-ray diffraction through transient resonances." Nature Communications 16.1 (2025): 847.
Ulmer, Anatoli, et al. "Nonlinear reversal of photo-excitation on the attosecond time scale”, accepted at Nature Communications (2026)

Speaker: Tais Gorkhover (Universität Hamburg/ DESY)

14:25 Ultrafast Resonant Single-Particle Imaging with Attosecond XFEL Pulses

Abstract
Ultrafast imaging enables the investigation of nanoscale dynamics in single particles using individual X-ray pulses. Accurate interpretation of such images requires understanding the strong interaction between the X-ray pulse and the sample, particularly when the electronic structure effects is of interest. We investigate ultrafast X-ray–induced dynamics in sodium iodide (NaI) nanoparticles using resonant singleparticle imaging with ultrashort X-ray free-electron laser pulses at the SwissFEL Maloja endstation. Photonenergy scans across the I 3d absorption edge were recorded to probe the pulse-length-dependent scattering response. Single-particle diffraction patterns were reconstructed by phase retrieval to extract scattering intensity and particle size. The measured scattering signals exhibit pronounced resonant features, with attosecond pulses providing a response closest to the ground-state electronic structure. For few-femtosecond pulses, the diffraction response is significantly modified, indicating the increasing influence of transient electronic excitations. Electronic structure simulations are employed to model X-ray–atom interactions and transient population dynamics, yielding time-dependent scattering cross sections. This combined approach provides insight into transient electronic configurations and defines the conditions under which ground-state resonant imaging can be achieved with attosecond XFEL pulses.

Acknowledgements
We acknowledge support from the Swiss National Science Foundation under grant number 197372 and the Paul Scherrer Institute for provision of SwissFEL beamtime at the Maloja endstation.

Speaker: Hankai Zhang (EPFL)

14:40 High-Throughput Single-Particle Imaging of Nonequilibrium Nanoparticle Ensembles Using Geometry-Constrained Reconstruction

The high repetition rate of X-ray free-electron lasers (XFELs) enables the collection of massive single-particle diffraction datasets, providing unprecedented opportunities to investigate nonequilibrium structural dynamics in nanoparticle ensembles. Here, we apply XFEL single-particle imaging to large ensembles of gold nanoparticles undergoing nonequilibrium processes including growth and plasmon-induced melting. The analysis reveals statistically significant distributions of transient particle geometries and evolving shape pathways beyond the capabilities of conventional ensemble-averaged characterization techniques, providing insight into the structural heterogeneity that emerges during dynamical transformations.

To efficiently extract structural information from these heterogeneous datasets, we develop a high-throughput analysis framework based on Monte Carlo sampling of particle shape and orientation parameter spaces under geometry-constrained reconstruction. The approach exploits the fact that nanoparticle structure is independent of overall particle orientation, allowing diffraction patterns from different particle views across the ensemble to collectively constrain otherwise weakly determined structural parameters. Geometric correlations learned from the full dataset further guide the reconstruction toward physically meaningful morphologies and enable rapid identification of subtle shape variations and transient structural features. The methodology has also been successfully transferred to multiple XFEL beamtimes and experimental configurations, demonstrating its broad applicability for high-throughput coherent imaging of heterogeneous nanomaterials.

These results establish geometry-assisted XFEL single-particle imaging as a powerful approach for probing nonequilibrium nanoscale dynamics and highlight the importance of integrating collective statistical information with advanced computational FEL imaging workflows.

[1] Shen, Zhou, et al. "Resolving nonequilibrium shape variations among millions of gold nanoparticles." ACS Nano 18.24 (2024): 15576–15589.

Speaker: Zhou Shen (Max-Planck-Institut für Struktur und Dynamik der Materie)

14:55 Fast Simulation of Wide-Angle Coherent Diffractive Imaging

Single-shot coherent diffractive imaging with intense XUV and soft X-ray pulses offers the exciting possibility of retrieving both the 3D structure and optical properties of nanoscale objects from a single diffraction pattern. Achieving this, however, requires an accurate description of the underlying wide-angle scattering and propagation effects, which are significantly more complex than in conventional X-ray diffraction.

I will introduce the propagation multi-slice Fourier transform method (pMSFT) [1], a fast and accurate approach for simulating the scattering process in this regime. Its derivation from first principles will be outlined, a unified physical picture to show its relation to existing methods is introduced, and finally systematic benchmarks demonstrating its superior performance for wide-angle scattering are presented.

[1] Laser Photonics Rev20, no. 7 (2026): e02001. https://doi.org/10.1002/lpor.202502001

Speaker: Paul Tuemmler (University of Rostock)

15:10 Coherent Correlation Imaging at XFELs: Enabling X-ray Imaging of Ultrafast Stochastic Processes

Synchrotrons and X-ray free electron lasers (XFEL) are intrinsically pulsed sources of light, which makes them ideal tools for time-resolved measurements. A wide range of ultrafast X-ray imaging techniques have been well established, and are now being widely utilized at numerous facilities worldwide. However, all of these methods require an averaging over an extended period of time (or a large number of pulse repetitions) to yield a high-quality image. Consequently, they are only capable of capturing highly reproducible and triggerable sample dynamics.
At the same time, many nanometer-scale systems exhibit fluctuations and stochastic processes, particularly in the presence of disorder. Recently, a new experimental method called Coherent Correlation Imaging (CCI) has been introduced [1]. This is a high-resolution, full-field microscopy technique that realizes multi-shot, time-resolved imaging of stochastic dynamics. The central idea of CCI is the classification of Fourier-space coherent diffraction patterns – even at a low photon count, where single-shot imaging is not feasible. This enables a selective averaging of the same-state frames, which are then converted into real-space images by a holographically-assisted iterative phase retrieval algorithm [2, 3].
In my talk, I will demonstrate the first implementations of CCI at XFELs that allow detection of ultrafast stochastic dynamics. We employ this new technique to resolve infra-red laser- and X-ray-induced nanometer-scale transformations of magnetic textures in ferromagnetic thin films. Our results establish CCI as a powerful, non-destructive imaging method, capable of revealing ultrafast stochastic phenomena that were previously inaccessible by conventional pump–probe or single-shot techniques.

[1] Klose, C. et. al. Nature 614, 256 (2023).
[2] Zayko, S. et al. Nat. Commun. 12, 6337 (2021).
[3] Battistelli, R. et al. Optica 11, 234 (2024).

Speaker: Boris Sorokin (University of Augsburg, Helmholtz-Zentrum Berlin, PSI - Paul Scherrer Institut)

15:25 Two-color X-ray Coherent Diffraction Movies

Capturing movies of ultrafast processes in nanomatter has been a driving force for the development of X-ray Free Electron Lasers (XFELs). Since the first demonstration of Coherent Diffraction Imaging (CDI) at an XFEL facility [1], the technique has moved from a niche proof-of-concept to a well-established, standard tool, allowing for snapshot imaging of fragile and transient structures, from quantum vortices in superfluid helium nanodroplets [2] to light-induced ultrafast melting [3]. Alongside the consolidation of established techniques, recent years have seen a substantial development of new operational modes for XFELs, opening exciting possibilities and calling for the development of novel experimental methods. Among these, modern XFELs are now capable of producing ultrashort, ultrabright pairs of X-ray pulses of two distinct "colors" (i.e., photon energies). The possibility of precisely controlling the time delay between the two colors down to the femtosecond regime allows for probing the same system at two different points in time, even down to the natural timescales of electron dynamics [4]. In this talk, we describe how two-color pulses can be combined with CDI experiments to capture two time-delayed snapshots of the same sample through a new technique called Dichography [5]. In particular, we show results on two-color diffraction patterns acquired at the European XFEL, where two images of the same xenon-doped superfluid helium nanodroplet, separated in time by up to 750 fs, are successfully retrieved despite demanding experimental conditions. While further improvements to both instrumentation and data analysis are necessary to maximize the resolution of the technique, the exciting possibility of two-color diffraction imaging at XFELs, so far considered unviable, opens a new research path for recording movies of ultrafast structural and electron dynamics at the nanoscale.

1. Chapman, Henry N., et al. "Femtosecond diffractive imaging with a soft-X-ray free-electron laser." Nature Physics 2.12 (2006): 839-843.
2. Gessner, Oliver, and Andrey F. Vilesov. "Imaging quantum vortices in superfluid helium droplets." Annual review of physical chemistry 70.1 (2019): 173-198.
3. Dold, Simon, et al. "Melting, bubblelike expansion, and explosion of superheated plasmonic nanoparticles." Physical review letters 134.13 (2025): 136101.
4. Hecht, Linos, et al. "Two-Color X-Ray Coherent Diffraction Imaging of Helium Nanodroplets." arXiv preprint arXiv:2508.19991 (2025).
5. Hecht, Linos, et al. "Dichography: two-frame ultrafast imaging from a single diffraction pattern." Nature Communications, in press (2026).

Speaker: Alessandro Colombo (ETH Zurich)

16:00 → 16:30 Coffee Break

16:00 → 16:30 FoE Award and Coffee

Convener: Gregor Knopp (PSI - Paul Scherrer Institut)

16:30 → 18:30 Nonlinear X-Ray

Convener: Gregor Knopp (PSI - Paul Scherrer Institut)

16:30 Solved and unsolved mysteries in EUV and x-ray transient gratings studies of coherent phonons and magnons

Laser-induced transient grating (TG) spectroscopy, in which two crossed short laser pulses produce a spatially periodic material excitation whose dynamics are monitored via diffraction of a time-delayed probe pulse, is a well-established optical technique used to study a wide range of phenomena. The extension of this technique into the EUV and x-ray spectral ranges accomplished in the past decade [1-4] enabled TG periods down to single-digit nanometers. EUV/x-ray transient gratings have been used to study nanoscale thermal transport as well as bulk and surface coherent phonons with nanoscale wavelengths [2,4-6]. A more recent application is studying nanoscale transient gratings of magnetization [7] including generation of coherent magnons [8,9]. In this talk, I will briefly review recent progress in the EUV/x-ray TG spectroscopy and discuss some solved and unsolved mysteries encountered in these studies.

[1] F. Bencivenga et al., Nature 520, 205 (2015).
[2] F. Bencivenga et al., Sci. Adv. 5, eaaw5805 (2019).
[3] J. R. Rouxel et al., Nat. Photonics 15, 499 (2021).
[4] H. Li et al., Nature Phys. 22, 483 (2026).
[5] A. A. Maznev et al., Appl. Phys. Lett. 119, 044102 (2021).
[6] D. Fainozzi et al., Phys. Rev. Lett. 132, 033802 (2024).
[7] D. Ksenzov et al., Nano Lett. 21, 2905 (2021).
[8] P. R. Miedaner et al., Sci. Adv. 10, eadp6015 (2024).
[9] P. R. Miedaner et al., Phys. Rev. Lett. 136, 176701 (2026).

Speaker: Alexei Maznev (MIT)

16:55 Four-wave mixing in the X-ray domain: Coherent nonlinear four-photon interaction with core-shell electrons

We present the experimental demonstration of coherent four‑wave mixing in the X‑ray regime (XFWM). We first demonstrate the process in neon near the K‑edge (870 eV) using single broadband SASE pulses from the Swiss Free Electron Laser [1]. A square spatial mask splits the incoming beam into three cut‑out beams arranged in a ‘folded BOX’ geometry, enabling background‑free signal detection. The beams intersect in a neon gas cell at a pressure of several hundred millibars, generating multi‑color resonant nonlinear signals involving Raman transitions, including X‑ray coherent anti‑Stokes (X-CARS) processes, which are characterized with an inline photon spectrometer. A rate‑equation model supports the interpretation of the measured XFWM signal. The extension of this experiment to molecular samples (O2, CO2 and N2O) addressing the K edges of O and N at 400 and 530 eV is also demonstrated, confirming the applicability of the method to molecular systems. In addition, we introduce approaches to extend the methodology to the time domain using the two‑colour mode available at the Athos branch of SwissFEL. In these preliminary experiments, we demonstrate the capability to generate time‑resolved signals arising from controlled stimulated emission in O2 following a Raman transition.
Finally, we discuss the perspectives to extend the discussed methodology to the attosecond regime, in line with a recent attosecond X-ray pump-attosecond X-ray probe experiment performed at the SQS instrument of EuXFEL. In this experiment, attosecond X‑ray interferometry was used to generate and probe site‑specific electronic wavepackets in acetic acid. By selectively exciting the two dominant pre‑edge resonances below the oxygen K‑edge with attosecond pump–probe pulses in a collinear transient‑absorption geometry, site‑selective bond excitation is demonstrated with attosecond resolution. The robustness of the setups, the strength of the signals, and the combined spectral, temporal and spatial information achieved in these experiments represent a significant step forward to establish a pathway towards multidimensional spectroscopy in the X‑ray domain.
References: [1] Morillo-Candas, et al. (2026) Nature, 649, 590–596

Speaker: Ana Sofia Morillo Candas (Max Planck Institute fur Kernphysik)

TEST.pdf

TEST.pptx

17:20 Orders of magnitude enhancement of magnetic soft X-ray transient grating signal at grazing incidence

Transient grating with EUV and soft X-rays has recently emerged as a powerful way to excite and detect nanoscale dynamics. Short penetration depths at these wavelengths have generally limited these experiments to thin films in a transmission geometry. In reflection, near normal incidence, the signal is entirely dominated by surface displacements. Here, we show that soft X-ray TG in a grazing incidence geometry enhances the sensitivity to permittivity changes, including electronic, elastic, and magnetic responses by up to four orders of magnitude in the soft X-ray regime. The enhancements for 1) refractive index gratings and 2) magnetic gratings are shown in Figs. A,B and Figs. C,D, respectively. We derive the angular dependence and enhancement of these contributions from Maxwell’s equations and detail important considerations for grazing incidence experimental geometries. Specifcally, we discuss the effect of grazing incidence on the pump efficiency, and important changes to the momentum conservation condition due to the presence of significant in-plane momentum at grazing angles. In addition, we show experimental results which demonstrate that at grazing incidence, magnetic responses can be on the same scale as surface displacement signals. These results greatly extend the applicability of soft X-ray TG to bulk samples, opening up many new avenues for studies of nanoscale phenomena. We envision that this geometry can be used, for example, to study charge transport in biomaterials, nanoscale magnetic dynamics in antiferromagnetic and altermagnetic crystals, and elastic dynamics on bulk and single-crystalline materials, to name a few.

Speaker: Peter Miedaner (MIT)

17:35 Nonlinear X-ray scattering reveals EUV-polariton

Spontaneous parametric down-conversion (PDC) of photons is a gateway into the quantum realm – thoroughly studied in nonlinear optics and ubiquitously used to generate non-classical states of light.
Here, we present a study of the effect in the x-ray regime and report the observation of a novel hybrid-state of light and matter that emerges from this fundamental nonlinear process. In our experiment, single x-ray photons are spontaneously converted by a diamond crystal into photon pairs. Of each pair, one photon is tuned to the extreme-ultraviolet (EUV) spectral range, where its coupling to the surrounding diamond is so strong that photonic and electronic properties hybridize:
This forms the EUV-polariton. Remarkably, the hybridization occurs without an enhancement cavity, which stands in stark contrast to the prevalent paradigm of strong-coupling in cavities.
The EUV-polariton links quantum hybridization in the microscopic domain to meso- and macroscopic length scales via its cavity-free propagation. This offers enticing prospects for studying buried interfaces and nanostructures using the polariton itself as a probe.

Speaker: Christina Boemer (DESY)

18:00 Spin Waves Excited by Hard X-Ray Transient Gratings

Ever brighter x-ray laser sources give rise to the possibility of using ultrafast hard x-rays to directly drive nanoscale lattice and magnetic dynamics. Recently, hard x-ray excitation of phonons was demonstrated using the x-ray transient grating (XTG) technique [1], but driving of magnetic dynamics in this regime has remained unexplored. In this work, we use XTG to excite magnetic precession in the insulating ferrimagnet gadolinium bismuth iron garnet (GdIG). Using Talbot imaging of a phase grating, x-ray photoionization occurs in a spatially periodic pattern. This results in the excitation of both bulk acoustic waves and a step-like periodic strain, which act as sources of torque on the spin system through inverse magnetostriction. Magnetization precession at the acoustic and spin wave frequencies were observed via diffraction of time-delayed optical pulse.[2] A micromagnetic model based on the Landau-Lifshitz-Gilbert equation corroborates that the spin waves were driven by x-ray generated strain. XTG is further established as a powerful technique to drive coherent nanoscale lattice and magnetic dynamics throughout the Brillouin zone.
[1] J. R. Rouxel et al., Hard x-ray transient grating spectroscopy on bismuth germinate, Nat. Photonics 15, 499 (2021).
[2] P. R. Miedaner et al., Spin Waves Excited by Hard X-Ray Transient Gratings, Phys. Rev. Lett. 136, 176701 (2026).

Speaker: Nadia Berndt (MIT Chemistry)

18:15 Portable nanofocusing optics for hard X-ray free-electron lasers

Tight focusing of XFEL pulses is a straightforward and effective approach to increase their fluence and peak intensity. At most XFEL facilities, nanofocusing optics are treated as permanent beamline components and installed in dedicated vacuum chambers to minimize environmental disturbances such as mechanical vibrations and thermal fluctuations. However, their fixed installation often limits experimental flexibility.

At SACLA, we are pursuing the development of portable total-reflection nanofocusing optics [1]. Rather than employing the conventional Kirkpatrick–Baez geometry, we utilize Wolter geometry [2,3], which provides significantly improved tolerance to incident-angle errors and environmental disturbances. The fabricated optics have been successfully deployed not only at SACLA but also at various experimental stations at LCLS and European XFEL.

In this presentation, I will introduce the design concepts and scientific applications of portable one-dimensional Wolter optics. In particular, I will focus on their use in exploration of nonlinear X-ray phenomena [2,4] and single-shot full-field imaging [5] at SACLA, European XFEL, and LCLS. I will also describe our ongoing development of monolithic two-dimensional portable Wolter optics aimed at achieving simpler alignment procedures, smaller focal spot sizes, and higher throughput.

[1] Y. Inubushi et al., J. Synchrotron Rad. 32, 534 (2025).
[2] J. Yamada et al., Nature Photonics 18, 685 (2024).
[3] I. Inoue et al., Optica 12, 309 (2025).
[4] I. Inoue et al., arXiv:2506.07968 (to be appeared in Nature Photonics)
[5] G. Yamaguchi et al., Nature Photonics 20, 721 (2026), T. Sato, in preparation.

Speaker: Prof. Ichiro Inoue (The University of Tokyo)

18:30 → 21:00 Poster Session (II)

Convener: All Posters

18:50 Experimental Observation of Four Wave-Mixing Processes Using Coherent Harmonics in the Extreme Ultraviolet Spectral Range

Abstract: Intense attosecond pulse trains generated by the seeded FEL FERMI have been temporally characterized using a covariance-based approach with single-shot photoelectron spectra [1,2]. Due to the high intensities achieved by FEL pulses, such attosecond pulse trains are ideal candidates for investigating nonlinear processes and, eventually, controlling them by varying the phase of individual harmonics composing the comb. In this work, we investigate third-order nonlinearities induced by an attosecond pulse train in a non-collinear four-wave mixing (FWM) process. We report the first experimental demonstration, to our knowledge, of third-order nonlinear signals generated by the combination of coherent extreme-ultraviolet (XUV) harmonics. Two time coincident replicas of multiharmonic XUV beams were crossed at an angle in a MgF$_2$ film to generate FWM signals [3,4]. Using self-diffraction as a probe of the nonlinear response spectrum, the Mg L$_{2,3}$ edge and several other resonant features were identified in the 50–70 eV range [5]. We observed and identified the FWM pathways of several nonlinear signals depending on multiple harmonics, with enhanced response when individual harmonics are resonant with one or more excitonic states in MgF$_2$. The results shows the presence of unintended neighboring harmonics generated by the FEL undulators, leading to new FWM pathways. We also discuss the possibility of using FWM processes for the temporal characterization of attosecond pulse trains.

[1] P. K. Maroju et al., Nature 578, 386 (2020).
[2] P. K. Maroju et al., New Journal of Physics 23, 043046 (2021).
[3] F. Bencivenga et al., Nature 520, 205 (2015)
[4] R. Mincigrucci et al., Nucl. Instrum. Methods Phys. Res. A 907, 132 (2018)
[5] Luca Assogna et al., Opt. Lett. 51, 1832–1835 (2026)

Speaker: Gopika Paduvil (Albert-Ludwigs-Universität Freiburg)

18:55 Ultrafast chemistry and biology at the SwissFEL Alvra instrument

Understanding photoinduced structural and electronic dynamics in chemical and biological systems requires experimental tools with femtosecond time resolution and sensitivity at the atomic scale. In this context, we describe the Alvra instrument at the Swiss X-ray free-electron laser (SwissFEL), which was developed to address these challenges by combining time-resolved X-ray spectroscopy and scattering techniques.
Alvra operates on the hard X-ray ARAMIS beamline and is primarily used to investigate ultrafast structural and electronic processes in photochemical and photobiological systems. It provides access to photon energies between 2 and 15 keV and integrates, within a single experimental environment, spectroscopic methods such as X-ray absorption (XAS), X-ray emission (XES), and resonant inelastic X-ray scattering (RIXS), together with structural techniques like X-ray solution scattering (XSS) and serial femtosecond crystallography (SFX). The setup is organized into two endstations, Prime and Flex, which together support experiments on liquids, crystalline samples, and solid-state systems, as well as user-built configurations.
One of the key aspects of Alvra is its ability to carry out time-resolved XES and RIXS in the tender X-ray regime, which is still not widely available at other XFEL facilities. In addition, advanced optical laser systems and shot-to-shot timing diagnostics enable pump probe experiments with temporal resolutions below 35 fs, under well-controlled conditions.
Since the beginning of user operation in 2019, a broad range of chemistry and biology experiments have been performed in Alvra. These studies have helped to understand the evolution of the transient electronic states and structural dynamics. As the user base grows and new methods are developed, the instrument continues to evolve in response to experimental needs.

Speaker: Claudio Cirelli (PSI - Paul Scherrer Institut)

19:00 Ultrafast Coherent Diffraction Imaging of Self-Assembled 1 nm Gold Nanoclusters at LCLS

Self-assembly of gold nanoclusters produces nano- to mesoscale structures with enhanced optical and electronic properties, making them promising building blocks for functional nanomaterials and plasmonic devices. These properties are commonly investigated using spectroscopic techniques, which typically provide only indirect information about the underlying structure. Directly characterizing the structural organization of such assemblies at the nanoscale therefore remains challenging.
Here we recorded single-shot coherent diffraction images at the Coherent X-ray Imaging (CXI) endstation of the Linac Coherent Light Source (LCLS) using intense, few-femtosecond X-ray pulses to investigate assemblies of 1 nm gold nanoclusters.
The recorded diffraction patterns reveal signatures of ordered arrangements within the nanocluster assemblies, extending from hundreds of nanometers up to several hundred micrometers. Such length scales are difficult to access with conventional techniques such as TEM or synchrotron SAXS/WAXS, which are limited by sample thickness, field of view, and scattering contrast. Numerical simulations reproduce the observed scattering features and provide additional insight into the structural organization within the assemblies. Our results demonstrate the capability of ultrafast coherent diffraction imaging to probe the structure of self-assembled nanocluster materials and pave the way for time-resolved studies of dynamic self-assembly processes.

Speaker: Jan Leutloff (Universität Hamburg)

19:05 Low Temperature Surface Diffraction Instrument of the Bernina Endstation

Experimental Setup of the Grazing Incidence chamber

Speaker: Mathias Sander (PSI - Paul Scherrer Institut)

19:10 Ultralow-temperature rare-earth magnetism measured at the SwissFEL Cristallina experimental station

Traditionally, neutron scattering has been the main technique to study spin order and excitations, such antiferromagnetism of LiErF4 [1]. However, exceptional improvements in terms of brilliance of accelerator-based X-ray sources have elevated X-ray scattering as an effective complementary technique that enables elemental sensitivity via electronic resonance effects, as well as the ability to probe small sample volumes.

Reaching sub-Kelvin temperatures at synchrotron X-ray sources is prohibitively difficult due the heat load on the sample from the incident continuous photon beam. This effectively precludes measurements at milli-Kelvin temperatures – hence we turned to pulsed X-ray FELs.

Here we report on the recently commissioned ultralow-temperature vector magnet setup that has recently become available to the user community at the SwissFEL Cristallina-Quantum endstation. We have used single X-ray FEL pulses for resonant magnetic X-ray scattering at the Er L3-edge (8.35 keV), showing that antiferromagnetic order of LiErF4 can be detected at sample temperatures as low as T = 35 mK before the onset of heating. Magnetism can also be tuned via vector magnetic fields, e.g. to lift the degeneracy of magnetic domains, and the effect on the magnetic order parameter directly monitored via the X-ray signal.

This proof-of-principle demonstration of electronic diffraction-before-destruction highlights the advantage of a brilliant, ultrafast X-ray source to measure fragile electron many-body states in the quantum criticality regime.

[1] C. Kraemer et al., Science, 336, 1416 (2012)

Speaker: Bill Francesco Pedrini (PSI - Paul Scherrer Institut)

19:20 ENGINEERING OXYGEN VACANCY CHANNELS IN BM-SCO FILM VIA STRAIN

Oxygen vacancies are not merely defects in materials but play a crucial role in determining their mechanical, optical, electrical, and catalytic properties.[1] Understanding and controlling these vacancies is essential for the development of advanced energy conversion and storage technologies.[2] SrCoO₂.₅ exhibits a brownmillerite structure with alternating layers of metal–oxygen octahedra and tetrahedra. When grown on SrTiO₃ (STO) substrates, the oxygen vacancy channels (OVCs) are typically aligned horizontally.[2] Tuning these OVCs strongly influences magnetic anisotropy and resistive switching behavior, which is important for memristor applications. In this work, BM-SCO thin films were deposited on Pb(Mg₁/₃Nb₂/₃)O₃–PbTiO₃ (PMN-PT) and BaTiO₃ (BTO) substrates using pulsed laser deposition. The lattice parameters of PMN-PT (4.0 Å) and BTO (3.994 Å) induce sufficient strain to transform horizontal OVCs into vertical ones without ionic liquid gating. The structural and magnetic properties of films grown on STO, PMN-PT, and BTO were systematically compared. Additionally in SRO/SCO/Nb-STO heterostructures, XRD measurements reveal a bias-induced topotactic phase transition from brownmillerite SrCoO₂.₅ to perovskite SrCoO₃ (Figure 2), driven by electric-field-controlled oxygen migration. XRD and magnetization (M–T and M–H) measurements confirm the orientation and evolution of OVCs. These tunable vacancy channels enable fast ionic transport, offering promising opportunities for energy storage, neuromorphic devices, and oxide electronics.[3]

Speaker: Rajan Mishra (UGC-DAE Consortium For Scientific Research,Indore M.P. India)

19:25 In-phase detection of magnetization dynamics with an X-ray free-electron laser

We have developed an all-electronic setup for synchronizing X-ray free-electron laser (XFEL) pulses in phase with microwave pulses at carrier frequencies up to 40 GHz, achieving sub-picosecond timing jitter. We demonstrate the capabilities of this synchronization scheme performing X-ray-detected ferromagnetic resonance experiments at an XFEL. In this experiment, conducted at the Maloja end-station of SwissFEL, the precession of the magnetization in the ferromagnet permalloy (Ni₈₁Fe₁₉) is excited by the magnetic field of a microwave resonator and probed by the X-rays exploiting the X-ray magnetic circular dichroism effect. This precessional motion was then revealed by scanning the microwave phase. This demonstration represents an important step toward implementing microwave pump experiments at XFELs and exploiting the femtosecond time resolution of XFEL pulses for detecting magnetization dynamics.

Speaker: Adrian Rutschmann (PSI - Paul Scherrer Institut)

19:30 Experimental confirmation of reduced electronic damage during X-ray diffraction with attosecond pulses

Over the past decade, intense femtosecond X-ray pulses generated by XFELs have greatly advanced structure determination of radiation-sensitive samples through the “diffraction-before-destruction” approach. The ultrashort duration of XFEL pulses allows diffraction data to be collected before significant structural changes occur, thereby enabling determination of atomic structures beyond the conventional radiation-dose limit.

A remaining challenge in XFEL-based measurements is the mitigation of X-ray-induced electronic excitation. When an XFEL pulse interacts with matter, photoionization occurs during the pulse, followed by Auger decay shortly thereafter. The emitted electrons collide with surrounding atoms and ionize additional bound electrons, initiating a cascade of collisional ionization. As a result, a large number of valence electrons become excited on a femtosecond timescale. This electronic damage drives the sample away from its pristine state [1] and limits the realization of truly damage-free X-ray measurements.

Attosecond XFEL pulses have the potential to overcome this limitation and enable more precise structure determination. Since a delay of several hundred attoseconds exists between the initial photoionization event and the onset of collisional ionization [2], the use of attosecond pulses is expected to substantially suppress X-ray-induced electronic excitation [3]. To investigate the feasibility of electronically damage-free X-ray diffraction using attosecond pulses, we employed silicon (Si) as a model system and evaluated the extent to which attosecond XFEL pulses from SACLA suppress electronic excitation through measurements of the Si 222 “forbidden” reflection. This reflection arises primarily from the non-spherical distribution of valence electrons. Therefore, its diffraction intensity serves as a sensitive probe of X-ray-induced electronic damage.

In this presentation, we will show the intensity dependence of the Si 222 reflection measured with attosecond and femtosecond XFEL pulses at SACLA and discuss the implications of attosecond pulses for damage-free X-ray diffraction experiments.

[1] I. Inoue et al., Phys. Rev. Lett. 131, 163201 (2023).
[2] I. Inoue et al., Phys. Rev. Lett. 126, 117403 (2021).
[3] V. Lipp, I. Inoue, B. Ziaja, in preparation.

Speaker: Dadian Guo (The University of Tokyo Graduate School of Frontier Sciences)

19:35 Accessing the spectral distribution of the diffracted light in a high-harmonic-generation based single-particle coherent diffraction imaging experiment with helium nanodroplets

Affiliation: Nanostructures and Ultrafast X-Ray Science, Laboratory for Solid State Physics, ETH Zurich
Email: cmichellod@student.ethz.ch

Authors : Carole Michellod, Indrani Dey, Isabelle Bolliers, Alessandro Colombo, Ehsan
Hassanpour-Yesaghi, Linos Hecht, Bruno Langbehn, Jannis Lehmann, Mario Sauppe,
Nora Sigrist, Janick Schrodt, Lea Schuepke, José Gòmez Torres, Frederic Ussling, Simon
Wächter, Björn Senfftleben and Daniela Rupp

Abstract : Single-particle coherent diffraction imaging (CDI) is a powerful imaging
method that enable to retrieve information about the structure and dynamics of
individual nanoparticles in free-flight from a diffraction pattern obtained in a single
illumination. High harmonics generation (HHG) sources producing highly intense
extreme ultraviolet (XUV) pulses has made single-shot single-particle coherent
diffraction imaging possible in a laboratory environment [1]. Recently, ultrafast electron
dynamics in individual helium nanodroplets have been traced with time-resolved HHG-
based CDI [2]. Accessing the ultrafast laser-driven electronic properties requires
spectral information of the diffracted light, as the interaction between the XUV and the
imaged nanoparticles is reflected in the spectral distribution. An exact shot-to-shot
knowledge of the diffracted spectrum is also of great importance for structural
reconstruction, since the fine details in the patterns are blurred by multiple harmonics
contributions. Here we present the development and first results of a simultaneous
diffraction imaging and diffraction spectroscopy experiment with helium nanodroplets.
We observe a change in the relative contributions of the different harmonics in the
diffracted spectrum of the XUV light, depending on the intensity of an NIR pulse
simultaneously illuminating the helium droplets, indicating transient laser-induced
changes in the optical properties of the helium droplets. Accessing the spectral
distribution of the diffracted light togerher with the single-shot diffraction patterns can
also unlock new opportunities for electron dynamics studies in FEL based experiments.

[1] Rupp, D., Monserud, N., Langbehn, B. et al. Coherent diffractive imaging of single
helium nanodroplets with a high harmonic generation source. Nat Commun 8, 493
(2017)
[2] Schäfer-Zimmermann J, Scheven T, Colombo A, Kolatzki K, Kruse B, Langbehn B,
Möller T, Monserud N, Sauppe M, Schütte B, et al. Laser-Induced Transient Opacity in
Helium Nanodroplets Probed by Single-Shot Coherent Diffraction. Ultrafast Sci. 6, 0146
(2026) DOI :10.34133/ultrafastscience.0146

Speaker: Carole Michellod (ETH Zurich)

19:45 Temporal diagnostics of hard X-ray attosecond pulses via amplified spontaneous emission

Attosecond science with ultrafast optical pulses has revolutionized our understanding of electron dynamics, enabling direct observation of charge migration, electron correlation effects, and field-driven phenomena on their intrinsic timescales. Recent advances in electron accelerator technologies at XFEL facilities have extended attosecond techniques into the hard X-ray regime [1,2], allowing matter to be probed with both attosecond temporal and angstrom spatial resolutions.

A key challenge in the generation and application of attosecond hard X-ray pulses is their temporal characterization. Current estimates of pulse duration rely on comparisons between measured spectra and theoretical simulations of the FEL process. However, such estimates are inherently uncertain, as the spectral width is strongly affected by the energy chirp of the lasing electron bunch and provides only a lower bound on the pulse duration.

In this presentation, we will present the direct experimental evidence of attosecond hard X-ray pulses, enabled by a new temporal diagnostics method [3]. Our approach leverages amplified spontaneous emission (ASE), which is a collective fluorescent process observable when tightly focused X-ray pulses irradiate 3d transition metal targets. By comparing the yield, angular divergence, and transverse correlation length of ASE to quantitative modelling of the ASE process, one can retrieve shot-to-shot pulse duration and transverse X-ray beam profile on the metal target.. We will discuss the underlying principles of this nonlinear diagnostic technique and its application to attosecond pulse characterization at European XFEL and SACLA.

[1] J. Yan et al., Nature Photon. 18, 1293 (2024).
[2] R. Robles et al., arXiv:2604.09969
[3] I. Inoue et al., arXiv:2506.07968

Speaker: Ichika Harima (Department of Applied Physics, School of Engineering, The University of Tokyo)

19:50 Time-delay-compensating monochromator beamline at FLASH

The pulse-length preserving double grating monochromator beamline FL23 at the XUV/soft x-ray free-electron laser FLASH went into user operation beginning of 2024. The beamline is designed to work in the soft X-ray regime covering the spectral range between 60-950 eV with a spectral resolving power of approximately 2000 [1]. A flexible microfocus at the experimental station is provided by a bendable Kirkpatrick-Baez mirror system while a femtosecond optical laser synchronized to the FEL enables experiments in a pump-probe scheme.
Here, we present the validation of the ultrafast beamline response utilizing sidebands from two-color above threshold ionization of atomic gas targets as well as spectral encoding measurements for different FL23 operation modes. The pulse duration and the pulse parameters were also verified with simulation which is presented on figure 1, where a) shows the measured trace and and b) shows the corresponding simulation result.

[1] L. Poletto, F. Frassetto, G. Brenner, M. Kuhlmann and E. Plönjes, Double-grating monochromatic beamline with ultrafast response for FLASH2 at DESY, Special Issue (PhotonDiag2017), J. Synchrotron Rad. 25, 131-137 (2018); https://doi.org/10.1107/S1600577517013777
[2] M. Tischer et al., Development of an APPLE-III undulator for FLASH-2. J. Phys.: Conf. Ser. 2380, 012017 (2022).

Speaker: Dr Balazs Nagyilles (DESY)

Thursday 24 September

08:30 → 10:30 Science Specific FEL Operation

Convener: TBD

08:30 Spectator-nuclei reveal nonlinear light-matter interaction driven by an X-ray free-electron laser (Tutorial)

the abstract, based on the word template given on this website, is provided in the attachment section of this page.

Speaker: Ralf Röhlsberger (Helmholtz Institut Jena)

09:10 New opportunities at XFELs for Mössbauer science

Mössbauer spectroscopy is well established across the natural sciences. The exceptionally narrow linewidths of Mössbauer nuclei (e.g. 4.7 neV for Fe57) also make them a promising platform for x-ray quantum optics. These narrow transitions feature long lifetimes, but on the other hand also allowed to only study single excitations for decades. This limitation changed with the advent of X-ray free-electron lasers (XFELs), which deliver orders of magnitude more resonant photons than synchrotrons. As a result, pulses containing multiple resonant photons occur frequently and the average resonant photon flux is increased, enabling experiments previously not possible. For example, multiple photon excitations and the subsequent dynamics [1, 2] as well as nuclear clock transitions [3, 4] can be studied. In addition, finite size effects in small samples can be explored and further progress of x-ray light sources, e.g. an X-ray free-electron laser oscillator, would enable a non-linear time evolution of the nuclei after the full excitation of a nuclear ensemble [5].

In this talk I will focus on the experimental progress related to the higher peak resonant photon flux. So far for Mössbauer spectroscopy extensive averaging was always necessary thereby limiting the applications. With up to 1000 resonant photons per shot at the European XFEL, we demonstrate a data-driven single-shot sorting of Mössbauer data [1], which opens up new applications for Mössbauer spectroscopy, e.g. fast time scales or different decay paths. In addition, I will present our results of interacting many-body simulations in the low-excitation regime at XFELs and beyond [5].

[1] M. Gerharz et al., arXiv:2509.15833 (2025)
[2] A. Chumakov et al., Nature Physics 14 261-264 (2018)
[3] Y. Shvyd’ko et al., Nature 622 471-475 (2023)
[4] P. Liu, M. Gerharz et al., arXiv:2508.17538 (2025)
[5] M. Gerharz, J. Evers, arXiv:2510.00970 (2025)

Speaker: Dr Miriam Gerharz (Max-Planck-Institut für Kernphysik)

09:35 Wave-packet manipulation via phase-locked schemes at a seeded free-electron laser

FERMI is the seeded free electron laser (FEL) facility in operation for international users within the Elettra – Sincrotrone Trieste research center. The adoption of a seeded design provides FERMI with the desirable qualities one expects from a laser: reproducibility, stability, spectral purity, synchronization, spatial and temporal coherence [1]. The latter implies that the electric field of the FEL pulse, not just its envelope, is accessible to the experimenter and can be used as a control parameter. Many viable schemes have been developed at FERMI over the years, that can be classified as chirping the carrier frequency [2], generating twin phase-locked pulses [3], phase-locking of fundamental and second [4] or third [5] FEL harmonics, phase-locking of consecutive harmonics of the seed laser [6]. In this talk I will present the common aspects of those schemes, and their use in AMO (atomic, molecular, optical) science experiments at the Low Density Matter beamline [7]; I will explore in detail the generation of twin pulses, and their application to the excitation and manipulation of a wave-packet of He Rydberg states [8].
The results originate from the joint effort of many international laboratories and of a large
number of researchers, whose work is gratefully acknowledged.
[1] E Allaria et al., Nature Photon. 6, 699 (2013); E Allaria et al., Nature Photon. 7, 913 (2013)
[2] D Gauthier et al., Phys. Rev. Lett. 115, 114801 (2015); F Richter et al., Nature 636, 337 (2024)
[3] D Gauthier et al., Phys. Rev. Lett. 116, 024801 (2016); A Wituschek et al., Opt. Express 28, 29976 (2020); A Wituschek et al., Nature Commun. 11, 883 (2020); D Uhl et al., J. Phys. Chem. Lett. 13, 8470 (2022); D Uhl et al., J. Phys. B 55, 074002 (2022); M Fushitani et al., J. Chem. Phys. 160, 104203 (2024)
[4] K C Prince et al., Nature Photon. 10, 176 (2016); D Iablonskyi et al., Phys. Rev. Lett. 119, 073203 (2017); M Di Fraia et al., Phys. Rev. Lett. 123, 213904 (2019); D You et al., Phys. Rev. X 10, 031070 (2020)
[5] M Žitnik et al., Optica 9, 692 (2022)
[6] P K Maroju et al., Nature 578, 386 (2020); P K Maroju et al., New J. Phys. 23, 043046 (2021); P K Maroju et al., Appl. Sci. 11, 9791 (2021); P K Maroju et al., Nature Photon. 17, 200 (2023); P K Maroju et al., Commun. Phys. 8, 207 (2025)
[7] V Lyamayev et al., J. Phys. B 46, 164007 (2013); C Svetina et al., J Synchrotron Radiat. 22, 538 (2015)
[8] M Dumergue et al., Phys. Rev. Res. 6, 043323 (2024)

Speaker: Carlo Callegari (Elettra - Sincrotrone Trieste S.C.p.A.)

10:00 X-ray chronoscopy in an optical pump – X-ray probe study

X-ray free electron lasers (XFELs) have revolutionized the study of matter, yet traditional optical pump - X-ray probe spectroscopy remains limited by arrival time jitter and probe pulse duration, often restricting temporal resolution to tens of femtoseconds. While current methods rely mainly on spectral changes and momentum transfer the ongoing development of time tools, for instance THz streaking setups [1], may soon enable new methodologies. This work elaborates on the recently conceptualized novel methodology, X-ray chronoscopy, that has potential to bypass the known constraints of ultrafast X-ray methods through precise analysis of the temporal profiles of incident and transmitted X-ray pulses [2,3].
In this work we developed numerical simulations employing a four-level atomic model and analyzed how optical pulses affect the X-ray chronoscopic signal, here defined as $-\ln\left( I_{1}\left( t \right) / I_{0}\left( t \right) \right)$ with time envelopes of the incident probe pulse $I_{0}\left( t \right)$ and the transmitted one $I_{1}\left( t \right)$ [4]. We studied the ZnSe system pumped with 5 fs-short 3.72 μJ pulses of 3.1 eV photons and probed with 0.1 μJ pulses of 9661.5 eV photons (equal to Zn$^{2+}$ K-edge binding energy). Different pump-probe delays were tested, probe pulse duration was tested in the range 5 - 100 fs and both pump and probe pulses were assumed monochromatic. The pump pulse time distribution was described with a Gaussian function and the probe one with a SASE-like function. We showed that the X-ray chronoscopic signal may be used to precisely determine relaxation rates in the material despite lifetimes being shorter than the probe pulse duration and even in the presence of significant pump–probe timing jitter.
Unlike conventional ultrafast methods requiring hundreds of pump-probe cycles, X-ray chronoscopy can capture the trace of electron dynamics within a single sequence of overlapping pump and probe pulses, provided the X-ray pulse duration exceeds the timing jitter and the dynamics in question. Our results indicate that by utilizing precise shot-to-shot temporal diagnostics, this approach can resolve electron processes in the single-femtosecond regime, offering a powerful complement to existing spectroscopy for investigating ultrafast phenomena like surface plasmon resonance and fundamental electron-phonon interactions.
[1] M. Wieland, P. Juranić et al., Optics Express 29, 32739 (2021).
[2] D. J. Bradley, Opt. Commun. 15, 231 (1975).
[3] W. Błachucki, C. Arrell, P. Juranić et al., Applied Sciences (Switzerland) 12, 1721 (2022).
[4] R. Fanselow, W. Błachucki, J. Szlachetko, X-Ray Spectrometry 55, 512 (2026).

Speaker: Dr Wojciech Błachucki (Institute of Nuclear Physics, Polish Academy of Sciences, Kraków, Poland)

10:15 Attosecond metrology at XFELs

Attosecond pulse modes at XFELs, such as those employing slotted-spoiler devices, low bunch charge, or photocathode shaping, are increasingly available at facilities around the world [1-3]. The intensity and photon energy of attosecond XFEL pulses greatly exceed those of conventional attosecond HHG pulses, granting the potential to study a much wider range of systems and processes than have hitherto been accessible. With greater access to these ultrashort pulses comes a greater need for means to exploit and characterise them; applying the established methods of attosecond metrology at XFELs is not straightforward, due to inherent difficulties with timing and phase stabilisation.

In recent years we have developed, demonstrated, and refined novel techniques to realise attosecond pump-probe streaking spectroscopy at XFELs. These self-referencing methods make pairs of measurements, with each pair using different phases of the same probe laser pulse, which provides sufficient information to reconstruct the instantaneous field parameters [4, 5].

The latest experiments incorporate dual electron time-of-flight detectors positioned at different points along the beam’s axis of propagation, resulting in a consistent, measurable Gouy phase shift between the two measurements in each pair. Here, we present preliminary results from these experiments, in which we measured the few-femtosecond delay between carbon 1s photoemission and Auger-Meitner emission in CF$_4$ and CH$_4$ – thereby contributing a time-resolved experimental answer to a long-running theoretical debate regarding the effects of an atom’s chemical environment on its relaxation dynamics. This measurement, facilitated by our novel technique, would not be possible with conventional attosecond streaking apparatus, and heralds the wide range of future possibilities for time-resolved ultrafast science at XFELs.

References:
1. Huang, S. et al. Generating Single-Spike Hard X-Ray Pulses with Nonlinear Bunch Compression in Free-Electron Lasers. Phys. Rev. Lett. 119, (2017).
2. Duris, J. et al. Tunable isolated attosecond X-ray pulses with gigawatt peak power from a free-electron laser. Nat. Photonics 14, 30–36 (2019).
3. Zhang, Z. et al. Experimental demonstration of enhanced self-amplified spontaneous emission by photocathode temporal shaping and self-compression in a magnetic wiggler. New J. Phys. 22, 083030 (2020).
4. Haynes, D. C. et al. Clocking Auger electrons. Nat. Phys. 17, 512–518 (2021).
5. Haynes, D. C. Extending attosecond techniques to X-ray free-electron lasers. Doctoral dissertation, Universität Hamburg (2022).

Speaker: Dr Daniel Charles Haynes (PSI - Paul Scherrer Institut)

10:30 → 11:00 Coffee Break

11:00 → 13:15 Science Specific FEL Operation

Convener: TBD

11:00 Lasing of a Cavity-Based X-ray source

Cavity-based X-ray Free Electron Lasers (CBXFELs) promise to transform the field of hard X-ray science by delivering radiation with exceptional brilliance, stability,and full three-dimensional coherence. At the European XFEL, we have recently achieved lasing with multi-pass gain in a proof-of-concept CBXFEL featuring a 132.8m round-trip hard X-ray cavity using diamond crystals in Bragg reflection.
This talk will present these results, and will classify them in the context of the development of CBXFEL sources and high brighntess X-ray pulses.

Speakers: Harald Sinn (EuXFEL), Patrick Rauer (Deutsches Elektronen-Synchrotron DESY)

11:25 Experimental Demonstration of Attosecond Soft and Hard X-ray Pulse Generation with Photocathode Shaping at SwissFEL

We demonstrate the generation of both soft and hard X-ray attosecond pulses at SwissFEL, based on the temporal shaping of the photocathode laser pulses [1]. Through flexible bunch compression schemes, the electron beam current profile evolves from a double-Gaussian shape with 10 ps duration into a femtosecond-scale isolated high-current spike in the core of a tens-of-femtosecond-long electron bunch. This high-current spike enables the generation of isolated soft and hard X-ray pulses characterized by a single spectral spike with multi-eV bandwidth, consistent with attosecond pulse durations. High-resolution measurements of the electron beam longitudinal phase space, together with start-to-end simulations, provide further support for the attosecond-scale duration of the emitted X-ray pulses. In the soft X-ray regime, the polarization of the attosecond pulses can be continuously tuned from linear to circular using the APPLE-X undulator [2]. We further demonstrate rapid switching of the soft X-ray pulse duration from sub-femtosecond to tens of femtoseconds by exploiting the time-dependent transverse phase space of the shaped electron beam.

Z.G., H.Z. and C.B. acknowledge support from the Swiss National Science Foundation under grant number 197372. We acknowledge the Paul Scherrer Institute for provision of SwissFEL beamtime.

[1] Zhang, Z., et al, New Journal of Physics 22.8 (2020): 083030.
[2] Schmidt, T., and Calvi, M., Synchrotron Radiation News 31.3 (2018): 35-40.

Speaker: Dr Zhaoheng Guo (École Polytechnique Fédérale de Lausanne (EPFL), PSI - Paul Scherrer Institute)

Cathode_Shaping_at_SwissFEL_Science_at_FEL2026.pdf

Cathode_Shaping_at_SwissFEL_Science_at_FEL2026.pptx

11:40 Cross-correlation measurements of the Two-Color Dual Oscillator IR FEL at FHI Berlin

The infrared free-electron laser (IR-FEL) at the Fritz Haber Institute recently commissioned a two-color upgrade, enabling simultaneous lasing with two IR FEL oscillators with independently tunable wavelength. This is realized by employing a high-frequency (500 MHz) kicker cavity to feeding alternating electron bunches from within the 1 GHz bunch train into the two separate IR FEL oscillators. Within this scheme, a high intrinsic temporal synchronization between both FEL outputs is expected1.
In our contribution, we present two-color time-resolved FEL-FEL cross correlation results proving this synchronization. We also confirm that the cross-correlation between both FELs broadens with increasing cavity detuning, in accordance with earlier studies*2. By leveraging the independently tunable pulse durations in each FEL, we further explore the achievable time-resolution that can be expected when the two-color, dual oscillator FEL is employed for future IR-IR pump-probe experiments.

*1. Schwettman, H. A., and T. I. Smith. "Two-color free-electron laser driven by a radio-
frequency linear accelerator." Journal of the Optical Society of America B 6.5 (1989): 973-
976.

**2. Bakker, R. J., et al. "Short-pulse effects in a free-electron laser." IEEE journal of quantum
electronics 30.7 (2002): 1635-1644.

Speaker: Akash Chandra Behera (Fritz Haber Institute of the Max Planck Society)

11:55 Special operation modes from Instrument side (HXRSS / Attoseconds / Twin Pulses)

Speaker: Joerg Hallmann (European XFEL)

12:15 Free electron lasers: a versatile tool to investigate metasurfaces

Metasurfaces, planar arrays of engineered subwavelength structures, enable unprecedented control over light’s amplitude, phase, and polarization, driving innovations in imaging, sensing, and active nanophotonics. For the optical characterization of these structures, tabletop optical parametric oscillators (OPOs) and amplifiers (OPAs) are widely used, as they provide rich information on linear light-matter interactions. However, pushing metasurfaces into extreme, non-perturbative regimes, particularly within the mid-infrared (mid-IR) and terahertz (THz) spectral windows, exposes fundamental physical bottlenecks in tabletop systems. Specifically, generating mid-IR/THz pulses via solid-state down-conversion is severely limited by the Manley-Rowe energy conservation constraint and the intrinsic phonon-absorption gaps (Reststrahlen bands) of the mixing crystals. Free-electron lasers (FELs) bypass these fundamental material limits. By utilizing relativistic free electrons undulating in a vacuum, FELs offer gapless tunability and natively high pulse energies across the entire polaritonic spectrum. Crucially, the unique macro/micropulse burst architecture of the FEL delivers the extreme peak fields required to trigger nonlinear phenomena, while allowing sufficient thermal relaxation to prevent the cumulative structural melting inherent to high-repetition-rate tabletop oscillators. These attributes make FELs uniquely suited for probing metasurfaces under extreme optical conditions. This perspective demonstrates how FEL-driven time-resolved pump-probe spectroscopy can be utilized to capture sub-picosecond carrier dynamics, plasmon dephasing, and biexponential electron-phonon relaxation. By leveraging these capabilities, researchers can map plasmonic hotspots, quantify fabrication-induced asymmetry, and assess the fundamental operational speed limits of ultrafast optical switches. Despite their limited global availability, FEL facilities offer transformative opportunities to advance metasurface science, providing a versatile platform for exploring the fundamental boundaries of light-matter interactions under extreme non-equilibrium conditions.

Speaker: Amirmostafa Amirjani (Quantum Solid-State Physics, Department of Physics and Astronomy, KU Leuven, Belgium)

12:30 X-ray FEL Science towards fusion (Tutorial part1)

Speaker: Thomas Cowan (HZDR)

12:50 X-ray FEL Science towards fusion (Tutorial part2)

Speaker: Harald Sinn (EuXFEL)

13:15 → 14:00 Lunch

14:00 → 16:15 Forum on Advanced FEL Techniques: Coherence

14:00 Introduction

Speaker: Marc Guetg (DESY)

14:10 Overview Talk coherence

Speaker: Markus Gühr (FLASH)

14:30 HXRSS

Speaker: Daria Gorelova (Brandenburg University of Technology Cottbus Senftenberg)

14:50 Evolution of FLASH1 into a Fully Seeded, MHz-Capable FEL Facility

Speaker: Eugenio Ferrari (Deutsches Elektronen-Synchrotron DESY)

15:10 Extension of FERMI FEL operation across the water window and beyond

The FERMI FEL, as an externally seeded facility, provides extraordinary temporal coherence, pulse-to-pulse stability, and spectral purity, overcoming the stochastic limitations inherent in self-amplified spontaneous emission (SASE). However, its maximum photon energy is primarily limited by the achievable harmonic multiplication before energy spread and microbunching degradation disrupt the seeding process. To extend the operational range of the FERMI FEL-2 line toward shorter wavelengths, three key enablers were implemented during a one-week campaign in May 2025: the minimization of the electron bunch slice energy spread, an increase in electron beam energy, and the use of a shorter seed wavelength. High spectral purity lasing on the FEL-2 line was achieved with energies of several microjoules down to the 80th harmonic (2.47 nm), near the oxygen K-edge; here, the final detected energy was further limited by losses within the optical transport system. Furthermore, by leveraging non-linear harmonic generation, the 1 keV photon energy limit was surpassed, demonstrating stable, fully coherent radiation at 1.1 nm. In this talk, I will present the technical details of this temporary upgrade, its limitations, and potential advancements to enable regular operation and user experiments in this extended short-wavelength regime.

Speaker: Peter Susnjar (ELLETRA)

Susnjar_ScienceAtFELs2026.pptx

15:30 DISCUSSION

16:15 → 16:45 Coffee Break

16:45 → 18:50 Forum on Advanced FEL Techniques: Tailoring Pulses

16:45 Tunable, phase-locked hard X-ray pulse sequences

Speaker: Philipp Dijkstal (PSI - Paul Scherrer Institut)

2026-09-24_PHLUX_PSI_Science_at_FELs_v2.pptx

17:05 Twin-Pulse user talk

Speaker: Cornelius Strom (DESY)

17:25 Short and Multicolor FEL Pulse Generation at ARIA@SPARC_LAB

ARIA will be the second free-electron laser beamline of the plasma-based accelerator facility EUPRAXIA@SPARC_LAB, hosted at Laboratori Nazionali di Frascati. Designed as a seeded FEL operating in the ultraviolet spectral range, ARIA will complement FERMI, providing radiation in the range 50-180nm.

Leveraging the capability of EUPRAXIA@SPARC_LAB to generate ultrashort electron bunches, we investigate the production of short, multicolor FEL pulses and the possibility of controlling their relative temporal delay. Different schemes are explored, including the use of dedicated undulator configurations as well as external optical delay lines. These approaches enable the generation of pulse pairs with different photon energy, polarization and delay, offering a versatile tool for time-resolved studies.

Based on both the simulation results presented here and the experience accumulated at FERMI, we expect these advanced multicolor and variable-delay operating modes to become a highly valuable and attractive feature for the user community.

Speaker: Giacomo Griseri (INFN-LNF)

ARIA-PSI.pptx

17:45 Coherent sub-femtosecond X-ray radiation: single-spike pulses and mode-locked frequency combs

Speaker: Eduard Prat Costa (PSI - Paul Scherrer Institut)

18:10 DISCUSSION

18:50 → 20:30 Barbeque / Grill

Joint Barbeque for participants of the Science@FELs Conference and the Forum of Advanced Technologies

Friday 25 September

08:30 → 10:30 Forum on Advanced FEL Techniques: Short Pulses

08:30 Overview short pulses

Speakers: Tais Gorkhover, Tais Gorkhover (Universität Hamburg)

08:50 XLEAP/LCLS/EuXFEL

Speaker: Agostino Marinelli (SLAC National Laboratory)

09:10 Generation of Ultra-short FEL Pulses via Laser-driven Local Bunch Compression

Speaker: Philipp Amstutz (DESY)

09:40 DISCUSSION

10:30 → 11:00 Coffee Break

11:00 → 12:50 Forum on Advanced FEL Techniques: New Ideas and FEL User concepts

11:25 Hard X-ray Free Electron Laser pulse multiplexing and steering capabilities at the European XFEL

Speaker: Mikako Makita (European XFEL GmbH)

11:50 Attosecond update

Speaker: River Robbles (SLAC)

12:10 Beating slippage in externally seeded FELs for fs pulses

Speaker: P. Rebernik (Elettra Sincrotrone Trieste)

12:30 closeout

Speaker: Giovanni Perosa (EuXFEL)

12:50 → 13:10 Final Remarks and Good Bye