# Science@FELs 2014

Europe/Zurich
WHGA/001 (Paul Scherrer Institute)

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#### Paul Scherrer Institute

5232 Villigen PSI
• Monday, 15 September
• 09:00 10:30
Student lectures 1 WHGA/001

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#### Paul Scherrer Institute

5232 Villigen PSI

Introductory lectures on FEL science

• 09:00
X-ray Free Electron Lasers: Seeing the Light Fantastic 45m
Speaker: Prof. Jerry Hastings (SLAC National Accelerator Laboratory)
• 09:45
High Energy Density Science with X-ray Free Electron Lasers 45m
Speaker: Prof. Justin Wark
• 10:30 11:00
Coffee break 30m WHGA/001

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#### Paul Scherrer Institute

5232 Villigen PSI
• 11:00 12:30
Student lectures 2 WHGA/001

### WHGA/001

#### Paul Scherrer Institute

5232 Villigen PSI
• 11:00
A New Era of X-ray Science: Beyond one-photon-at-a-time 45m
Speaker: Joachim Stöhr
• 11:45
X-ray Free Electron Lasers - A bright future for crystallography 45m
Speaker: Ilme Schlichting (Max Planck Institute for Medical Research)
• 12:30 13:30
Lunch 1h OASE

### OASE

#### Paul Scherrer Institute

5232 Villigen PSI

In PSI restaurant OASE

• 13:30 14:00
Opening session WHGA/001

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#### Paul Scherrer Institute

5232 Villigen PSI
• 14:00 15:50
Condensed Matter 1 WHGA/001

### WHGA/001

#### Paul Scherrer Institute

5232 Villigen PSI
• 14:35
Imaging ultrafast demagnetization dynamics after a spatially localized optical excitation 25m
Ultrashort, coherent x-ray pulses of a free-electron laser are used to holographically image the femtosecond magnetization dynamics within a magnetic domain pattern after creation of a localized optical excitation. The time resolved experiment was carried out at the DIPROI beamline at the free electron Laser facility FERMI in Trieste, Italy. Circularly polarized, coherent, 100 fs long, soft x-ray pulses tuned to the M-edge of Cobalt at 20.8 nm were used to image the magnetic domain pattern via Fourier Transform holography exploiting x-ray circular magnetic dichroism. The sample consisted of a magnetic Co/Pd multilayer on a gold mask with an elliptical object hole (2.4x1.2µm²) and reference holes with 50 nm diameter. The synchronized infrared pump pulses, incident under 45 deg., were reflected and focused by the curved shape of the elliptical object hole leading to a standing wave with a pronounced electric field enhancement with a spatial extent of approx. 150x150nm². With a sub-100 nm spatial resolution we observed a laterally conﬁned reduction of the magnetization within a couple of hundred femtoseconds followed by its recovery on a slower time scale. Additionally, the experimental results show evidence of a spatial evolution of magnetization, which we attribute to ultrafast transport of non-equilibrium spin polarized electrons for early times and to a ﬂuence dependent re-magnetization rate for later times. C. von Korff Schmising et al., PRL 2014 (in press)
Speaker: Dr Clemens von Korff Schmising (Technische Universität Berlin)
• 15:00
Direct structural characterization of photo-induced coherent phonon oscillations in BaFe2As2 via ultrafast x-ray diffraction 25m
Insight into the relationship between the intertwined lattice, spin and orbital degrees of freedom is vital for the understanding of high-temperature superconductivity in iron-based materials. We address this question by using ultrafast x-ray diffraction at the Linac Coherent Light Source (LCLS) to measure the time-evolution of a lattice Bragg peak in photo-excited BaFe2As2. Upon excitation with a femtosecond optical laser pulse, we observe an ultrafast increase and oscillation of the Bragg peak intensity. The frequency of this oscillation is 5.5 THz, which is consistent with the coherent excitation of an A1g phonon mode. This mode modulates the As-Fe-As bond angle that is crucial for determining the underlying electronic structure and also correlated with the superconducting transition temperature. We estimate the variation of the bond angle in this photo-induced coherent state by modeling the scattering form factor of the Bragg peak in the presence of the A1g phonon mode. The influence on the electronic and magnetic degree of freedom in this photo-excited coherent oscillatory state will be discussed. Our observations provide a direct and unique view on the dynamics of lattice degrees of freedom, which cannot be studied by other means.
Speaker: Dr Simon Gerber (Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory)
• 15:50 16:20
Coffee break 30m WHGA/001

### WHGA/001

#### Paul Scherrer Institute

5232 Villigen PSI
• 16:20 18:10
Condensed Matter 2 WHGA/001

### WHGA/001

#### Paul Scherrer Institute

5232 Villigen PSI
• 16:20
Coherent control and electrical detection of charge excitations in hydrogenic silicon impurities with a Free Electron Laser 35m
Speaker: Prof. Ben Murdin
• 16:55
Terahertz spectroscopy of zero- and two-dimensional semiconductor nanostructures with the free-electron laser FELBE 25m
The free-electron laser facility FELBE in Dresden, a unique source of intense, quasi-cw, nearly transform-limited ps pulses in the mid-infrared and terahertz (THz) regimes, provides unique research opportunities. In high-quality semiconductor quantum wells, we investigate the dynamics of excitons, i.e. two-dimensional, hydrogen-like electron-hole quasi-atoms. Tuning FELBE in resonance with the transition between the excitonic 1s and 2p states (at ca. 2 THz) allows us to study the dynamics of intra-excitonic population transfer. Moreover, strong terahertz pumping results in a characteristic Rabi splitting of the 1s exciton state, which is a manifestation of the intra-excitonic Autler-Townes effect. In semiconductor quantum dots, resonant THz excitation between different sublevels is shown to produce an absorption contrast in aperture-less scattering scanning near-field optical microscopy (s-SNOM). This effect allows us to obtain functional s-SNOM images with deep sub-wavelength resolution, where the contrast originates from far-infrared absorption by single electrons. Quantum dots are also known to have very long electronic relaxation times caused by a reduced phase space for optical phonon scattering. We will report on THz four-wave mixing experiments demonstrating that the associated electronic coherence times approximately equal the population relaxation time at low temperatures. This property makes quantum dots promising for quantum optical applications at THz frequencies.
Speaker: Dr Harald Schneider (Helmholtz-Zentrum Dresden-Rossendorf)
• 17:20
TIME-RESOLVED PHOTOELECTRON SPECTROSCOPY USING HIGH-REPETITION RATE FREE-ELECTRON LASERS 25m
Free electron lasers (FEL) are unique X-ray sources regarding pulse duration, coherence properties and peak brilliance. Their potential to study ultrafast dynamics has been demonstrated in time-resolved x-ray spectroscopy experiments. Time-resolved photoelectron spectroscopy (TR-PES) is a very valuable tool to study non-equilibrium electron dynamics of condensed matter systems. However, due to the ultra-short, very intense FEL pulses the number of photoelectrons per pulse is limited by space charge considerations. The signal-to-noise ratio in these experiments is governed by the detection efficiency and the repetition rate of the photon source. The superconducting FEL's FLASH at DESY and European XFEL operate at very high repetition rates, which is perfectly suited for TR-PES. Statistical fluctuations of the FEL’s require single shot detection for ultimate time- and energy resolution. To make use of the high angular acceptance of time-of-flight electron spectrometers this translates in the necessity to have fast multi-hit capable time- and spatially resolving detectors. Examples for time-resolved XPS from experiments at FLASH illustrating the opportunities to follow ultrafast charge rearrangement in solids and at surfaces will be discussed. New developments in terms of spectrometers and detectors for TR-PES with high-repetition rate FEL’s will also be presented. This work is supported by the BMBF within the priority programs FSP 301 and 302.
Speaker: Prof. Wilfried Wurth (Universität Hamburg/DESY)
• 17:45
A time-dependent order parameter for photo-induced ultrafast phase transitions 25m
The exploration of the subtle interaction of structural and electronic degrees of freedom in strongly correlated electron systems on the femtosecond time scale is an emerging area of research. One goal of these studies is to advance our understanding of the underlying correlations, another is to find ways to control on an ultrafast time scale the technological relevant properties of these materials. Perovskite manganites, prototypical examples of strongly correlated materials, exhibit properties such as colossal magnetoresistance and insulator-to-metal transitions that are intrinsically related to symmetry changes of the atomic lattice and to fascinating ordering patterns of the spins, charges and orbitals. Here we report on a recent study on the dynamics of photoexcited epitaxial film of Pr0.5Ca0.5MnO3 using femtosecond resonant x-ray diffraction. Taking advantage of the high flux of the LCLS free electron laser we are able to access different classes of superlattice reflections, each with sensitivity to different components of the phase transition. We find that although the actual change in crystal symmetry associated with this transition occurs over different time scales characteristic of the many electronic and vibrational coordinates of the system [1-3], the dynamics of the phase transformation can be well described using a single time-dependent ‘order parameter’ that depends exclusively on the electronic excitation.
Speaker: Dr Paul Beaud (Paul Scherrer Institut)
• 18:30 20:00
Poster session SLS

### SLS

#### Paul Scherrer Institute

5232 Villigen PSI

Poster sesion with Apéro Riche

• Tuesday, 16 September
• 09:00 10:50
Biology WHGA/001

### WHGA/001

#### Paul Scherrer Institute

5232 Villigen PSI
• 09:00
New opportunities with Free Electron Lasers to study membrane protein dynamics 35m
Speaker: Gebhard Schertler (Paul Scherrer Institut)
• 09:35
Radiation damage in serial femtosecond crystallography 25m
Serial femtosecond crystallography (SFX) is an emerging technique for protein structure determination. SFX has been proposed as the method of choice for data collection of highly radiation sensitive systems such as nanocrystals in general and e.g. metalloproteins. Previous high resolution SFX measurements performed at the LCLS do not show any signs of radiation damage at a dose of <100 MGy. However, analyzing smaller and smaller samples will require higher flux densities resulting in GGy doses. Our recent damage experiments at LCLS explored this high dose regime using X-ray emission spectroscopy (XES) showing pulse length and pulse fluence dependent ionization of iron. High dose SFX experiments on a small protein containing two iron-sulfur clusters show distinct damage of the two clusters both below and above the iron K absorption edge (7.1 keV). These experimental results give insights into radiation induced damage processes in biological macromolecules that contain high Z centers at high intensity X-ray sources.
Speaker: Dr Karol Nass (Max Planck Institute for Medical Research)
• 10:00
Fixed Target 2D and 3D Protein Crystallography at XFELs 25m
Over the last years, serial femtosecond nanoscrystallography (SFX) has been demonstrated successfully in a number of experiments at LCLS and SACLA. Most SFX appllications to date have used three-dimensional (3D) nano- or microcrystals and have utilized a liquid jet based sampe introduction approach. That approach typically requires large amounts of sample and is not conducive to measuring two-dimensional (2D) protein crystals. 2D crystallography of membrane proteins has been developed originally for cryo electron microscopy and is an avenue for obtaining structural information on membrane proteins that do not easily form 3D crystals necessary for traditional x-ray crystallography. Here we describe a fixed target approach for 2D and 3D crystallography at XFELs that allows x-ray diffraction measurements on samples supported by thin substrates at room temperature. We present first promising results from experiments at LCLS that included 2D crystal samples of the membrane protein bacteriorhodopsin and 3D microcrystal samples of REP24, a soluble protein. We discuss strategies for reducing amounts of sample required and increasing speed of data acquisition further to render this approach a viable aternative to the liquid jet based sample introduction approaches. The fixed target approach is expected to open up new opportunities for time-resolved SFX on samples that are not abundant and/or require the sample to be flat.
Speaker: Dr Matthias Frank (Lawrence Livermore National Laboratory)
• 10:25
Macromolecular Crystallography at XFELs and LCLS: Current Status, Limitations and Future Plans 25m
One of the early success stories of XFELs is the technique of Serial Femtosecond Crystallography (SFX), where crystals are illuminated by the x-ray pulses one at a time and the biomolecular structure is deduced from integrating all these measurements. After a few years of development, the technique has reached some level of maturity. In this talk I will quickly review the state of macromolecular crystallography at XFELs and at LCLS in particular, both for 3D and 2D crystals. With a few years of operation of LCLS behind us, some issues and limitations with the SFX technique have been identified. These include technical issues such as sample delivery and sample quantities, software limitations and fundamental issues with the beam parameters such as the spectrum of the SASE beam. I will discuss limitations and present results from experiments that have attempted to address these issues. For example, the use of the seeded FEL beam for crystallography will be discussed. Finally, future plans for LCLS in the area of macromolecular crystallography will be presented, including a plan for a new LCLS endstation as well as the use of multiplexing and serial operation of multiple stations at LCLS. Also, as LCLS moves towards as high repetition rate machine, along with the European XFEL, recent results using high speed imaging of exploding liquid jets will be presented. These results have direct implication on the ultimate usable repetition rates for XFELs with liquid jets.
Speaker: Sebastien Boutet (Linac Coherent Light Source)
• 10:50 11:10
Coffe break 20m WHGA/001

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#### Paul Scherrer Institute

5232 Villigen PSI
• 11:10 13:00
Chemistry WHGA/001

### WHGA/001

#### Paul Scherrer Institute

5232 Villigen PSI
• 11:10
Ultrafast surface chemistry at LCLS 35m
Speaker: Prof. Martin Beye
• 11:45
Observing molecular dynamics with ultrafast X-ray spectroscopies and scattering 25m
Ultrafast structural dynamics is an emerging field aiming to deliver a detailed understanding of the elementary steps in reacting chemical species, which involve changes in their nuclear, electronic and spin states. Such processes are vital ingredients in chemistry and biology, but also in technological applications, including efficient charge transport in light harvesting molecules and ultrafast switchable molecular magnets. In order to unravel this complex dynamic behavior we have implemented a suite of ultrafast X-ray spectroscopic and scattering tools to zoom into both the electronic and nuclear structures, with the goal to ultimately deliver a molecular movie of ongoing chemical processes. In view of the many potential applications in chemical and biological dynamics it is desirable to increase the signal-to-noise (S/N) level of such experiments as well as to decrease the time resolution into the femtosecond time domain. We present our benchmark results using a versatile setup that permits simultaneous measurements of ultrafast X-ray absorption and emission spectroscopies combined with X-ray scattering, which has been recently implemented by us at different synchrotrons and XFELs. We applied it to study different photochemical reactions, ranging from nascent radicals in solution, molecular spin transitions and ligand exchange reactions to photocatalytic systems, with the goal to deliver a deeper understanding of the elementary steps in chemical reactivity.
Speaker: Dr Wojciech Gawelda (European XFEL)
• 12:10
Electron Rearrangement Dynamics in Dissociating Multiply Charged Iodine Molecules 25m
We use an XUV-pump--XUV-probe scheme to access electron rearrangement dynamics in dissociating molecular iodine ions. A first pulse of 87 eV, delivered by the free-electron laser FLASH, multiply ionizes and consequently fragments the iodine molecules (I$_2$). Depending on its delay with respect to the pump pulse, the identical probe pulse induces electron transfer between the dissociating ions, which results in symmetrically charged fragments. In contrast, for large delays electron transfer is blocked and we observe asymmetrically charged ion pairs. By means of a reaction microscope we record the yield of coincident ion pairs from dissociating I$_2^{n+}$ molecular ions as a function of the time delay. This allows to determine the critical internuclear distances and corresponding time scales up to which electrons transfer is possible for various molecular break-up channels. Our results are in very good agreement with predictions of a classical over-the-barrier model demonstrating its validity in an energy regime relevant for FEL, plasma and chemistry applications.
Speaker: Kirsten Schnorr (Max-Planck-Institut für Kernphysik)
• 12:35
A combined experimental and theoretical study of the excited state dynamics of a prototypical Cu(I)-phenanthroline complex 25m
Cu-phenantholine complexes are promising candidates for photoelectronic applications. However, despite being extensively studied since the early '80s, a complete understanding of their photophysical and photochemical properties is still lacking. In particular, both the mechanism by which luminescence quenching occurs in donor solvents and the nonadiabatic relaxation processes in the low lying singlet and triplet MLCT excited states are hotly debated. I will present a combined time-resolved X-ray absorption spectroscopy and theoretical study of a prototypical Cu(I)-phenanthroline complex [Cu(dmp)2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline), aimed at elucidating the excited state mechanisms occurring within the femto- to nanosecond time domains. In the pico and nanosecond domain, time-resolved XAS does not confirm the previous assignment of excited state lifetime shortening in donor solvents (acetonitrile) to a metal centred exciplex. Instead, the lifetime quenching of the title complex and related complexes may be rationalised by the solvent induced decrease of the triplet MLCT energy. In the femtosecond time domain, we employ the Multi Configurational Time Dependent Hartree method to explore the nonadiabatic relaxation of [Cu(dmp)2]+ following excitation into the S3 state. The results, in conjunction with previous experiments allow us to rationalise the ultrafast dynamics of this complex and shed insight into the relaxation of the system to the long live triplet state.
Speaker: Thomas Penfold
• 13:00 14:00
Lunch 1h WHGA/001

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#### Paul Scherrer Institute

5232 Villigen PSI

PSI restaurant OASE

• 14:00 15:50
Theory WHGA/001

### WHGA/001

#### Paul Scherrer Institute

5232 Villigen PSI
• 14:00
Transitions in matter triggered by intense ultrashort X-ray pulses 35m
Speaker: Prof. Beata Ziaja-Motyka
• 14:35
Soft X-ray ionisation of atoms within TDDFT and nuclear effects on the attosecond TRPES of ethylene 25m
First, I will demonstrate the capabilities of time-dependent density functional theory (TDDFT) for strong-field, short wavelength (soft X-ray) physics, as compared to a formalism based on rate equations. TDDFT provides a very good description of the total and individual ionization yields for Ne and Ar atoms exposed to strong laser pulses over a wide range of intensities. Second, using TDDFT we examine the energy, angular and time-resolved photoelectron spectra (TRPES) of ethylene in a pump-probe setup. Studying the photoemission spectra provides us direct access to the dynamic evolution of the molecule’s electronic levels. Further, by including the nuclei's motion, we provide direct chemical insight into the chemical reactivity of ethylene.
Speaker: Mrs Alison Crawford (Nano Bio Spectroscopy Group, UPV/EHU)
• 15:00
XMDYN: Modeling radiation damage of XFEL irradiated samples 25m
High-resolution x-ray imaging of nanosize biological samples is one of the most important goals of the research with X-ray free electron lasers (XFEL) [1]. Outstanding results have already been achieved in serial nano-crystallography [2]. There has been also a significant progress in the field of single particle imaging [3]. However, radiation damage is still a limiting factor, in particular for non-periodic objects. This requires thorough theoretical investigations of the time evolution of the irradiated samples. Here we report on XMDYN [4], our molecular-dynamics based tool to model the dynamics of finite samples irradiated by high intensity x-ray pulses. First we describe the theoretical approach used. To validate the model we then show predictions of the model as compared to experimental results. New developments such as the on-the-fly connection to the atomic physics XATOM toolkit [5] enabling accurate treatment of heavy elements and a possible code extension towards large-scale calculation are also discussed. [1] R. Neutze, R. Wouts, D. van der Spoel, E. Weckert and J. Hajdu, Nature 406, 752 (2000) [2] L. Redecke et al. Science 339, 227 (2013). [3] M. M. Seibert et al. Nature 470, 78 (2011) [4] Z. Jurek, B. Ziaja, and R. Santra, XMDYN (CFEL, DESY, Hamburg, Germany, 2013). [5] S.-K Son and R. Santra, XATOM an integrated toolkit for X-ray and atomic physics. (CFEL, DESY, Hamburg, Germany, 2011).
Speaker: Zoltan Jurek (Center for Free-Electron Laser Science, DESY; Hamburg Centre for Ultrafast Imaging)
• 15:25
Atomic kinetics in solids under strong XFEL irradiation 25m
The role of elementary processes during interaction of short XUV free-electron laser pulses with solid material is studied by means of a specific collisional radiative model (CRM) without spatial dimension applied to solid aluminum. The calculation of the energy deposition requires a proper model for the bound electron kinetics as the pulse is mainly absorbed via photoionization of bound electrons. We have modified a classical CRM with detailed atomic physics to reproduce the density effects of continuum lowering and account for the processes involving the valence band. It allows us to treat the transition from solid to warm dense matter. The free-electrons, initially in the valence band,are described by a Fermi-Dirac distribution and are coupled to the bound electrons kinetics through collisional processes. We found that the absorption is highly non-linear with respect to the photon energy and the XFEL intensity. The main elementary processes at stake are thephotoionization, the Auger decay, the three-body recombination and the collisional ionization. A change in the XFEL parameters modifies the competition between these processes and thus, the electron kinetics and the absorption. We also studied the expansion of the matter after irradiation with the code X-RIM coupling detailed configuration kinetics to a 1D Lagrangian hydrodynamics treatment. The study of 0D and 1D calculations allows us to identify the different relevant mechanisms responsible for the matter evolution.
Speaker: Mr Basil Deschaud (CELIA - Université de Bordeaux)
• 15:50 16:20
Coffee break 30m WHGA/001

### WHGA/001

#### Paul Scherrer Institute

5232 Villigen PSI
• 16:20 18:10
Site visit PSI areal

### PSI areal

#### Paul Scherrer Institute

5232 Villigen PSI
• 18:30 22:00
Conference Dinner OASE

### OASE

#### Paul Scherrer Institute

5232 Villigen PSI
• Wednesday, 17 September
• 09:00 10:50
Matter under extreme Conditions WHGA/001

### WHGA/001

#### Paul Scherrer Institute

5232 Villigen PSI
• 09:00
Ultra-intense X-ray laser matter interaction with hard X-ray free electron laser 35m
• 09:35
Resolving Ultrafast Heating of Dense Cryogenic Hydrogen 25m
We performed an XUV-pump XUV-probe experiment on warm dense hydrogen with sub-picosecond time resolution at the XUV free-electron laser facility (FLASH) at DESY (Hamburg). Ultra-fast impulsive electron heating was initiated by a ≤ 300 fs short photon pulse at 92 eV energy. A second XUV pulse probed jitter-free the heated sample via x-ray scattering at variable time delays. We showed that the initial molecular structure dissociates within (0.9 ± 0.2) ps. This allowed to infer the energy transfer rate between electrons and ions. We evaluated Saha and Thomas-Fermi ionization models in radiation hydrodynamics simulations, predicting plasma parameters that were subsequently used to calculate the static structure factor. A conductivity model for partially ionized plasma was validated by two-temperature density functional theory coupled to molecular dynamic simulations, and agreed with our experimental data. Our results [1] provide important insights and the needed experimental data on transport properties of dense plasmas. [1] U. Zastrau et al., PRL 112, 105002 (2014)
Speaker: Dr Sven Toleikis (DESY)
• 10:00
Multiphoton induced x-ray fluorescence of Fe atoms 25m
We report on multiphoton processes involving inner-shell electrons of solid Fe with very intense and ultra-short hard x-ray free-electron laser (XFEL) pulses. The experiment was carried out at the CXI end-station of the Linac Coherent Light Source by means of the high energy resolution x-ray emission technique. The XFEL beam of ~10^12 photons/pulse and pulse energy in the range of 1-4 mJ was focused on the solid Fe sample. The ultra-focused x-ray beam provided an extreme fluence (~10^4-10^5 photons/Å^2). Moving the sample out of the focus along the beam allowed varying the fluence. For the K x-ray emission spectra measurements the bent crystal von Hamos x-ray spectrometer of PSI [1] installed at CXI and equipped with the CSPAD detector developed at SLAC was employed. To explore the nonlinear interaction of Fe atoms with high-fluence XFEL radiation the photon beam energies were chosen below the Fe K-shell single- and double-ionization thresholds. The K x-ray emission spectra comprising the Kα (K^{-1}-->L^{-1}) diagram lines and the rich satellite structures due to the multiphoton induced multiple ionization, as well as the Kα^h hypersatellite (K^{-2}-->K^{-1}L^{-1}) transitions, were measured as a function of the XFEL fluence. The obtained results evince the nonlinear two-photon processes leading to K-shell ionization, and the K-shell hollow atom formation following sequential two-photon absorption. [1] J. Szlachetko et al., Rev. Sci. Instrum. 83, 2012, 103105.
Speaker: Dr Joanna Hoszowska (Department of Physics, University of Fribourg, CH-1700 Fribourg, Switzerland)
• 10:25
Electron spin resonance study in the chiral ferrimagnet Cu2OSeO3 using pulsed magnetic fields up to 64 T and terahertz free electron laser 25m
The recent discovery of skyrmions in Cu2OSeO3 has established a new platform to create and manipulate skyrmionic spin textures. We use high-field electron spin resonance (ESR) spectroscopy combining a terahertz free electron laser and pulsed magnetic fields up to 64 T to probe and quantify its microscopic spin-spin interactions. Besides providing direct access to the long-wavelength Goldstone mode, this technique probes also the high-energy part of the excitation spectrum which is inaccessible by standard low-frequency ESR. Fitting the behavior of the observed modes in magnetic field to a theoretical framework establishes experimentally that the fundamental magnetic building blocks of this skyrmionic magnet are rigid, highly entangled and weakly coupled tetrahedra.
Speaker: Dr Mykhaylo Ozerov (Dresden High Magnetic Field Laboratory (HLD), Helmholtz-Zentrum Dresden-Rossendorf, D-01328, Germany)
• 10:50 11:10
Coffee break 20m WHGA/001

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#### Paul Scherrer Institute

5232 Villigen PSI
• 11:10 13:00
Atomic Molecular Systems WHGA/001

### WHGA/001

#### Paul Scherrer Institute

5232 Villigen PSI
• 11:10
Complementary users of high-harmonic and FEL-based XUV radiation 35m
Speaker: Prof. Marc J. J. Vrakking (Max-Born-Institut Berlin)
• 11:45
Hidden Charge States in Soft-X-Ray Laser-Produced Nanoplasmas Revealed by Fluorescence Spectroscopy 25m
Highly charged ions are formed in the center of composite clusters by strong free-electron laser pulses and they emit fluorescence on a femtosecond time scale before competing recombination leads to neutralization of the nanoplasma core. In contrast to mass spectrometry that detects remnants of the interaction, fluorescence in the extreme ultraviolet spectral range provides fingerprints of transient states of high energy density matter. Spectra from clusters consisting of a xenon core and a surrounding argon shell show that a small fraction of the fluorescence signal comes from multiply charged xenon ions in the cluster core. Initially, these ions are as highly charged as the ions in the outer shells of pure xenon clusters with charge states up to at least 11+. Subsequent electron transfer and thermalization followed by nonradiative recombination produce significantly lower charge states within a few hundred femtoseconds. Thus, a sacrificial tamper layer provides an efficient electron source for partial neutralization of highly charged ions created in the center. The significant reduction of charge states increases the available time for recording a diffraction pattern in coherent imaging experiments.
Speaker: Andreas Przystawik (DESY)
• 12:10
Dichroism in the two-color multi-photon ionization of helium 25m
The combination of intense femtosecond X-ray and NIR pulses produced by Free Electron Lasers (FEL) and synchronized optical lasers, respectively, offers various opportunities to gain detailed insight into atomic photoionization dynamics. After an early study on the linear dichroism in the two-color photoionization of helium at FLASH [1], we have now taken advantage of the availability of circularly polarized XUV radiation at FERMI to explore the circular dichroism and its influence on the photoionization dynamics [2]. The combined XUV and optical fields give rise to the formation of so-called sidebands in the photoelectron spectrum. The variation of the intensity of these sidebands, when switching the helicity of one of the light fields, is defined as circular dichroism in the photoionization and provides unique information on the partial photoionization cross sections, i.e. on the relative importance of the angular momentum of the outgoing electrons. In addition, the comparison of the experimental data with the results of theoretical models describing the two-color photoionization process eenables us to determine the degree of circular polarization of the FEL beam as well as the sign of the helicity, i.e. parameters, which are generally difficult to extract by other means. Possibilities for future applications, including resonant phenomena, will be outlined and discussed. [1] M. Meyer et al., Phys. Rev. Lett. 101, 193002 (2008) [2] T. Mazza et al . Nat.Comm. 5, 3648 (2014)
Speaker: Dr Michael Meyer (European XFEL)
• 12:35
Cluster Experiments at the Free Electron Laser FERMI at Elettra 25m
The ionization dynamics of atomic clusters in intense ultrashort laser pulses has been an active area of research in recent years at near-infrared (NIR), vacuum-ultraviolet, and soft x-ray wavelengths. The dynamics of multi-component nanoplasmas turns out to be interesting model systems for realizing single-shot ultrafast diffraction imaging of large biomolecules in the x-ray domain. Helium nanodroplets are particularly attractive for studying strong field ionization due to the simple electronic structure and due to the homogeneous density distribution of He droplets.Furthermore, the property of He droplets to pick up dopant atoms which aggregate either in the droplet interior or at the droplet surface makes it possible to study heterogeneous systems. Recent experiments of doped helium droplets performed at the LDM beamline [1] at the XUV Free-Electron-Laser FERMI at Elettra are presented, demonstrating the importance of highly collective processes driving nanoplasma formation [2,3]. Furthermore, water clusters are probed by XUV-UV pump-probe spectroscopy in order to study fundamental dynamics of electron hydration and atmospheric chemistry processes. The solvation dynamics triggered by the XUV FEL leads surprisingly to 3 isomers of the hydrated electron having individually different formation times. [1] V. Lyamayev, et. al, J. Phys. B 46 164007 (2013) [2] A. LaForge, et al., Scientific Reports, 4, 3621 (2014) [3] Y. Ovcharenko, et al., Phys. Rev. Lett. 112, 073401(2014)
Speaker: Prof. Frank Stienkemeier (University of Freiburg)
• 13:00 14:00
Lunch 1h WHGA/001

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#### Paul Scherrer Institute

5232 Villigen PSI

PSI restaurant OASE

• 14:00 16:00
New Developments WHGA/001

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#### Paul Scherrer Institute

5232 Villigen PSI
• 14:00
Revival of X-ray cross-correlations "because" of X-ray Free Electron Lasers 30m
Speaker: Bill Francesco Pedrini (Paul Scherrer Institut)
• 14:30
Arrival time diagnostic using free carriers generation induced by X-ray FEL 30m
Speaker: Dr Marion Harmand
• 15:00
Coherent imaging by X-ray laser diffraction 30m
Speaker: Prof. Yoshinori Nishino
• 15:30
Advanced pump-probe experiments at FERMI, results and perspectives 30m
Speaker: Prof. Filippo Bencivenga