15-17 September 2014
Paul Scherrer Institute
Europe/Zurich timezone

Students Session
X-Ray Free Electron Lasers: Seeing the Light Fantastic
Jerome Hastings (SLAC National Accelerator Laboratory, Menlo Park, California, USA)

Free electron lasers (FELs) spanning wavelengths from below 0.1 nm to beyond 20 nm are now operational worldwide and more are under construction. For those operating in the 0.1 nm range we are at the beginning of a new world. We will review the properties of these fantastic light sources, how they have been applied, how they continue to evolve and what might be awaiting just ahead. Slides

A New Era of X-ray Science: Beyond one-photon-at-atime
Joachim Stöhr (SLAC National Accelerator Laboratory, Menlo Park, California, USA)

My talk will discuss the basic x-ray interactions with matter with emphasis on resonant processes. I will start with the classical and quantum mechanical description of x-rays and how we can picture them in a unified framework. I will then discuss the new concepts required to describe X-FEL pulses and their interactions with matter, and how these interactions differ from those described by the powerful Kramers-Heisenberg perturbation theory, that has served us so well during the first 100 years of x-ray science. In particular, I will discuss how we describe non-linear x-ray interactions in atoms and solids, and at what x-ray fluences they become important. I will also present a dream experiment associated with chemical reactivity and energy transfer and what beam parameters and instrumentation are required to advance the entirely new field of non-linear x-ray science. Slides

Condensed Matter Session
Coherent control and electrical detection of charge excitations in hydrogenic silicon impurities with a Free Electron Laser

Ben Murdin (University of Surrey, UK)

Shallow donor impurities in silicon, once frozen out at low temperature, share many properties in common with free hydrogen atoms [1]. It is only very recently that it has been possible to investigate the time-domain dynamics of orbital excitations such as the 1s to 2p [2-7], due to the difficulty of obtaining short, intense pulses in the relevant wavelength range. The Free Electron Laser is ideal. We have measured the population dynamics [2,4] of electrons orbiting around phosphorus impurities in commercially- available silicon, and shown that the lattice relaxation lifetime is about 200ps, only 1 order of magnitude shorter than the radiative lifetime of free hydrogen. In the cleanest silicon possible, isotopically pure Si-28, it is even longer [5]. Recently we demonstrated that the FEL can be used to resonantly transfer electrons from one donor to another [6] and that the excited population can be detected electrically [7]. Most exciting is the demonstration that the excitations have long coherence lifetime for quantum information applications [3]. Slides

[1] BN Murdin et al Nature Communications 4, 1469 (2013)
[2] NQ Vinh et al, Proc Nat Acad Sci USA 105, 10649 (2008)
[3] PT Greenland et al Nature 465, 1057 (2010).
[4] NQ Vinh et al Phys Rev X 3, 011019 (2013)
[5] HW Hubers et al Phys Rev B 88, 035201 (2013)
[6] K Litvinenko et al Phys. Rev. B 89, 235204 (2014)
[7] E Bowyer et al Appl Phys Lett 105, 021107 (2014)

Imaging ultrafast demagnetization dynamics after a spatially localized optical excitation
Clemens von Korff Schmising, Stefan Eisebitt, Michael Schneider, Christian Günther (Technische Universität Berlin, Germany), Emanuele Pedersoli, Nicola Mahne, Flavio Capotondi (Elettra-Sincrotrone Trieste, Italy), Jan Lühning, Jonathan Perron, Boris Vodungbo (Sorbonne Universités, UPMC Univ Paris 06, France), Bastian Pfau (Lund University, Sweden), Leonard Müller (DESY, Hamburg, Germany)

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)]. Slides

DiProI, the coherent diffraction imaging end-station at FERMI@ Elettra FEL user facility: present status and future research opportunities
Flavio Capotondi (Elettra Sincrotrone, Trieste, Italy)
The multipurpose measurement station for CDI, operating at the DiProI beamline of the FERMI@Elettra XUV/soft X-ray FEL, is designed to meet the users’ requirements for performing a wide range of static and dynamic studies [1], and has been open to the users since 2012. The flexible design permits to exploit the state of the art performance of the seeded FERMI FEL source [2,3]. This presentation will overview the different class of experiments already performed, which include single-shot CDI [1,4] time-resolved magnetic holography [5,6], two-color FEL pumpprobe diffraction [7] and interferometric phenomena observed in FEL-induced transient IR reflectance and transmittance [8]. The emphasis will be on demonstrating the new research opportunities for studies of transient states of matter with high temporal resolution opened thanks to the implemented almost jitter-free
pump-probe schemes [7,9] that allow for time-resolved experiments using multi-color twin FEL pulses [7] or IR/FEL [6,8] configurations. The shorter wavelengths, down to 4 nm in the first lasing harmonic, soon available with FERMI-2 [2], will expand the instrument capabilities to CDI experiments with free-standing samples, injected in to the FEL interaction region as well dynamic studies reaching the L2,3-adsorption edges of magnetic transition metals in the third harmonic. Implementation of grazing geometry and 3D set-ups is under consideration and development, which will allow new class of exotic experiments. Slides

Biology Session
X-ray free electron laser studies on membrane protein 2D crystals
Ching-Ju Tsai, Celestino Padeste, Bill Francesco Pedrini, Guido Capitani, Gebhard Schertler, Xiao Dan Li (Paul Scherrer Institut, Villigen, Switzerland); Matthias Frank (Lawrence Livermore National Laboratory, Menlo Park, California, USA)

The intense and ultrafast ulses generated by X-ray free electron lasers (XFEL) permit new methods to study the structure and dynamic of proteins. So far structural studies of proteins using XFELs has been limited to proteins that form well-ordered nm-to-μm sized 3D crystals. Here we present methods to prepare 2D crystals for XFEL measurements and XFEL diffractions from 2D crystals. By adopting concepts used in electron microscopy we were able to collect diffractions of two different membrane proteins: proton pump bacteriorhodopsin and the voltage-gated sodium channel NavCt. For best results the 5nm thick and 0.5-2μm wide crystals were stabilized by a solid support of wafer covered with either a 30-50 nm thick silicon nitride membrane or a 20-50 nm thick layer of carbon film. To prevent crystal drying in the vacuum of the sample chamber, crystals were embedded in sugars such as glucose or trehalose that are commonly used in electron microscopy. Optimized sample preparation resulted in diffractions up to 6A resolution with the same unit cell reported from electron microscopic studies. This proof-of-principle study demonstrates the promise of XFEL for the structural analysis of membrane proteins in the native-like lipidic environment of 2D crystals. In comparison to 3D crystals, larger conformational freedom of proteins embedded in 2D crystals and easy, fast accessibility for interacting ligands may facilitate future time-resolved studies of this important class of proteins. Slides

Radiation damage in serial femtosecond crystallography
Nass Karol (Max Planck Institute for Medical Research)

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. Slides

Theory Session
Transitions in matter triggered by intense ultrashort X-ray pulses
Beata Ziaja-Motyka (CFEL, Hamburg, Germany)

In my talk I will give an overview on the recent results of our theoretical investigation how the unique properties of X-ray free-electron laser (FEL) radiation can be employed to modify extended atomic or molecular assemblies, and to create new states of matter. I will discuss three topics that are related to various irradiation regimes that can be achieved, depending on the FEL pulse fluence and its wavelength: (i) radiation-induced transitions in solids, (ii) modeling of nanoplasmas created from finite systems, and (iii) atomic processes within laser-created plasmas and warm-dense-matter. Slides

Soft X-ray ionisation of atoms within TDDFT and nuclear effects on the attosecond TRPES of ethylene
Alison Crawford (Nano Bio Spectroscopy Group, UPV/EHU, Spain)

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. A. Crawford Soft X-ray ionisation of atoms within TDDFT and nuclear effects on the attosecond TRPES of ethylene. Slides

XMDYN: Modeling radiation damage of XFEL irradiated samples
Zoltan Jurek (Center for Free-Electron Laser Science, DESY, Hamburg Centre for Ultrafast Imaging, Germany), Beata Ziaja (Center for Free-Electron Laser Science, DESY, Hamburg Centre for Ultrafast Imaging, Institute of Nuclear Physics, Polish Academy of Sciences), Robin Santra (Center for  Free-Electron Laser Science, DESY, Hamburg Centre for Ultrafast Imaging, University of Hamburg, Germany)

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 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. Slides

[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).

Atomic, Molecular-Systems Session
Complementary uses of High-Harmonic and FEL-based XUV radiation
Marc Vrakking (Max Born Institute, Berlin, Germany)

In the last few years two novel XUV/x-ray sources have acquired an important role within atomic and molecular physics research, namely high-harmonic generation (HHG) and a new generation of free electron lasers (FELs). These two sources are of significant interest for at least two reasons. On the one hand, the short optical period of XUV/x-ray light allows the synthesis of attosecond laser pulses that can be used to study electronic processes on their natural timescale. On the other hand, the availability of very energetic photons provides opportunities for the development of novel spectroscopic techniques that are based on diffraction and/or the use of inner shell excitation. Free electron lasers and high-harmonic sources each present users with their own specific advantages and disadvantages. In my talk I will present a number of recent examples where we have exploited the complementarity of the novel HHG and FEL light sources. I will present experiments on XUV ionization of aligned molecules that access the nodal structure of the orbitals that are ionized, and that reveal the onset of the manifestation of the structural information, as well as experiments on the ionization and fragmentation of large rare gas clusters under the influence of intense XUV light pulses. Slides

Dichroism in the two-color multi-photon ionization of helium
Michael Meyer, Tommaso Mazza, Markus Ilchen, Nikolay Kabachnik (European XFEL, Hamburg, Germany), Carlo Callegari, Paola Finetti, Oksana Plekan, Kevin Prince, Marcello Coreno, Robert Richter (Elettra Sincrotrone Trieste, Italy), Patrick O’Keeffe, Lorenzo Avaldi, Paola Bolognesi (CNR IMIP, Bari, Italy), Kiyoshi Ueda (Tohoku University, Japan), John Costello (Dublin City University, Ireland), Elena Gryzlova, Alexei Grum-Grhzimailo (Lomonosov Moscow State University, Russia), Andrej Kanzansky (Donostia International Physics Center, Donostia- San-Sebastián, Spain)

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 enables 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. Slides

[1] M. Meyer et al., Phys. Rev. Lett. 101, 193002 (2008)
[2] T. Mazza et al . Nat.Comm. 5, 3648 (2014)

New Developments Session
Advanced pump-probe experiments at FERMI, results and perspectives

Filippo Bencivenga (Elettra Sincrotrone, Trieste, Italy)

The unique features of the FERMI seeded FEL source [1] have been exploited to extend advanced table-top methods into the XUV/soft x-ray domain [2]. In this context, we demonstrated a two-colour seeded FEL emission with selectable time and wavelength separation between the two pulses.3 We used this new tool in a FEL-pump/FELprobe diffraction experiment at the Ti M-edge at the expected time (≈0.5 ps) for the transition towards the warm dense matter regime [3,4] More recently, we carried out a transient grating experiment in which the FEL pulses were split and recombined on a SiO2 sample at a finite crossing angle [5]. The induced dynamic Science@FELs 2014 69 XUV grating was probed in a pump-probe (“jitter-free”) [6] fourwave- mixing scheme by an optical pulse coming from the seed laser. The coherent non-linear interaction of the three pulses originated a detectable coherent beam propagating along the phase matching direction. The time dependence of the signal bears evidences of three distinct oscillating features having periods of about 0.2 ps, 1 ps and 22 ps. The latter is consistent with longitudinal acoustic phonons having a wavevector equal to that of the induced XUV grating. Such non-linear XUV/soft x-ray wave-mixing experiments will be further developed at the facility in a user-dedicated beamline (EIS-TIMER), [7] which will be operational in 2015. In this frame, the use of multi-colour, fully coherent FEL emission would open up the possibility to develop more sophisticated four-wave-mixing methods [2,4]. Slides

[1] E. Allaria et al., Nature Photonics 6, 699 (2012)
[2] F. Bencivenga et al., New Journal of Physics 15, 123023 (2013)
[3] E. Allaria et al., Nature Communications 4, 2476 (2013)
[4] F. Bencivenga et al., Faraday Discussions, DOI: 10.1039/c4fd00100a (2013)
[5] F. Bencivenga et al., unpublished
[6] M. B. Danailov et al., Optics Express 22, 12869 (2014)
[7] F. Bencivenga and C. Masciovecchio, Nucl. Instr. and Meth. A 606, 785 (2009)