Science@FELs 2020

14 Sept 2020, 08:00 → 16 Sept 2020, 18:00 Europe/Berlin

Christian Bressler (European XFEL), Elke Plönjes (DESY), Martin Beye (DESY), Serguei Molodtsov (European XFEL)

Due to the ongoing COVID-19 pandemic the Science@FELs 2020 conference was held as an online conference from 14-16 September 2020.

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The International Science@FELs Conference took place from 14-16 September, 2020 as a set of online sessions. As fifth conference in the Science@FELs series, it followed those in Stockholm in 2018, in Trieste in 2016, at the Paul Scherrer Institute in 2014, and at DESY in 2012. 

Science@FELs is organised as a regular biannual activity of FELs OF EUROPE, the collaboration of European FEL and SPS Facilities.

Science@FELs 2020 focussed on scientific highlights achieved during the last years at FELs and laser facilities world-wide.

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In cooperation with

Monday, 14 September

1 Welcome and opening remarks

13:55 Break

Session 1 - Coherent imaging

2 High repetition rate single-particle imaging at the European XFEL

The dream of imaging single molecules was instrumental to the construction of X-ray free-electron lasers (XFELs). The European XFEL marks the beginning of the high-intensity, high-repetition-rate and high data-rate era of XFELs, bringing the dream closer to reality. In this talk, I will present the evolution of X-ray diffraction imaging and in particular highlight the latest results from the European XFEL. I will also demonstrate the importance of developing robust structure validation procedures for the long-term success and wider adoption of the method as well as how to best make use of this wealth of data to extract as much new knowledge as possible. I will also discuss what new techniques might be over the horizon and what is still required to achieve the dream of ultrafast X-ray diffractive imaging of single proteins.

Speaker: Dr Filipe Maia (Uppsala University)

3 Quantum Imaging with incoherently scattered X-rays from a Free-Electron Laser

For more than 100 years, X-rays have been used in crystallography to determine the structure of crystals and molecules via coherent diffraction methods. With the advent of accelerator-driven free-electron lasers (FEL) new avenues for high-resolution structure determination are explored that go even far beyond conventional X-ray crystallography [1-3]. Yet, these techniques rely on coherent scattering, i.e., incoherence due to fluorescence emission or wave front distortion is generally considered detrimental for these approaches. Here we show that using methods from quantum optics, i.e., exploiting spatial higher order photon correlation functions, 1D, 2D and even the full 3D arrangement of sources that scatter incoherent X-ray radiation can be resolved [4-8]. We discuss a number of properties of this incoherent diffraction imaging method that are conceptually superior to those of conventional coherent X-ray structure determination and point out that current FELs are ideally suited for the implementation of this approach [7]. We also present an experimental demonstration in the soft X-ray domain, where we use higher-order photon correlation functions to achieve higher fidelities in the image reconstruction and potentially sub-Abbe resolution [8]. Figure 1: Illustration of incoherent diffraction imaging: A large number of diffraction snapshots of incoherent X-rays scattered by a 3D source arrangement is recorded by a CCD; the photon correlations of each snapshot are determined individually; averaging over many snapshots leads to a pattern that yields the initial 3D distribution of the sources. References 1. H. N. Chapman et al., Nature Phys. 2, 839 (2006). 2. H. N. Chapman et al., Nature 470, 73 (2011). 3. A. Barty, J. Küpper, H. N. Chapman, Annu. Rev. Phys. Chem. 64, 415 (2013) and references therein. 4. C. Thiel, T. Bastin, J. Martin, E. Solano, J. von Zanthier, G. S. Agarwal, Rev. Lett. 99, 133603 (2007). 5. S. Oppel, T. Büttner, P. Kok, J. von Zanthier, Phys. Rev. Lett. 109, 233603 (2012). 6. A. Classen, F. Waldmann, S. Giebel, R. Schneider, D Bhatti, T. Mehringer, J. von Zanthier, Phys. Rev. Lett. 117, 253601 (2016). 7. A. Classen, K. Ayyer, H. N. Chapman, R. Röhlsberger, J. von Zanthier, Phys. Rev. Lett. 119, 053401 (2017). 8. R. Schneider et al., Nature Phys. 14, 126 (2018); see also News & Views, Nature Photon. 12, 6 (2018

Speaker: Joachim von Zanthier (Uni Erlangen)

15:00 Break

Session 2 - Biology Applications

4 Ultrafast Electronic and Structural Dynamics of Heme Proteins Probed by Time-resolved X-ray Spectroscopy at XFELs

Iron-containing heme proteins are amongst the most important known proteins and the metal-binding center is paramount to the function of these systems. Much effort has been dedicated to the study of the structure, function and dynamics of these proteins. Time-resolved X-ray spectroscopy is a particularly well-suited tool for this purpose given that the element specificity provides a direct and sensitive probe of the dynamics from the metal-binding center point of view. This work focuses in the investigation of light-induced ultrafast electronic and structural dynamics of two important heme proteins, Nitrosyl Myoglobin (MbNO) and Cytochrome C (CytC), by time-resolved x-ray absorption (XAS) and x-ray emission (XES) spectroscopies. Upon visible photoexcitation of the heme group, MbNO undergoes dissociation of the ligand which is accompanied by a spin change and a structural reconfiguration of the porphyrin ring. Part of the excited population undergoes recombination in multiple timescales through an intermediate state that is presumed to be a high spin domed ligated form of MbNO. We carried out a time-resolved XES experiment and the results offer new insight on the dissociation-recombination dynamics and capture all the intermediate spin states involved in the relaxation from the initially excited heme back to the ground state. Meanwhile, CytC we focused on the investigation of the nature of the relaxation process fowling excitation of the heme in its ferric form. Ferrous heme proteins (such as the MbNO) are known for undergoing dissociation of the axial ligand, however, this is not observed in their ferric counterparts. This has led to a long discussion of the relaxation pathway involved in these types of systems, which was believed to proceed entirely via vibrational cooling back to the ground state. We recently performed fs-XAS and fs-XES experiments that challenge this interpretation evidencing the presence of heme doming and de-excitation via high spin states.

Speaker: Dr Camila Bacellar (PSI)

5 Untangling the sequence of events during the S-state transitions in Photosystem II

The water oxidation reaction in nature occurs in Photosystem II (PS II), multi-subunit protein complex, in which the Mn4CaO5 cluster catalyzes the reaction. The reaction comprises four (meta)stable intermediates (S0, S1, S2 and S3) and one transient S4 state, which precedes dioxygen formation occurring in a concerted reaction from two water-derived oxygens bound at the OEC. This reaction is coupled to the two-step reduction and protonation of the mobile plastoquinone QB at the acceptor side of PS II. Using serial femtosecond X-ray crystallography (SFX) and simultaneous X-ray emission spectroscopy (XES) with multi-flash visible laser excitation at room temperature, we studied all (meta)stable states with resolutions of 2.04-2.08 Å. We also collected some timepoint data between the S-states in order to understand the sequence of events. The current status of this research and the mechanistic understanding of the water oxidation reaction based on the X-ray techniques is presented. Reference: J. Kern, et al., Structures of the intermediates of Kok’s photosynthetic water oxidation clock, Nature, 563, 421 (2018). M. Ibrahim, et al., Untangling the sequence of events during the S2 → S3 transition in photosystem II and implications for the water oxidation mechanism, PNAS, 117, 12624 (2020).

Speaker: Dr Junko Yano (Lawrence Berkeley National Laboratory)

16:05 Break

Session 3 - Femto-chemistry & Catalysis

6 Tracking the ultrafast dynamics of photoinduced spin-state switching in metallogrid complexes

Photoinduced spin-switching in molecular complexes containing d4-d6 transition metal ions is an important phenomenon that enables numerous applications in chemistry, biology and nanotechnology. At the microscopic level, this process originates from the bistability between a low-spin (LS) and a high-spin (HS) state, which can be activated through light irradiation. Although the vast majority of the building blocks tend to be mononuclear complexes, unique electronic and magnetic properties emerge when the nuclearity of the complexes increases from monomeric to oligomeric. Fundamental diagnostics about the photo-switching event in a polynuclear assembly are embedded, not only in the spin transition rate, but also in the structural rearrangements that result from photoexcitation. Although these multi-scale dynamics are essential for understanding the spin-switching, they remain ill-characterized for large systems due to their strong coupling. This talk will present time-resolved X-ray measurements performed on a family of pyrazolate-based [2x2] FeII grid complexes, both on the picosecond and on the femtosecond time scales. The direct monitoring of the ultrafast dynamics with atomic and spin sensitivity reveals the intrinsic nature of their switching mechanism.

Speaker: Dr sophie canton (eli-alps)

7 Femtosecond X-ray absorption reveals excited state structural dynamics in cobalamins

The fate of a photoactive molecule is determined by the electronic and structural rearrangements that follow excitation. Femtosecond (fs) X-ray free electron lasers (XFELs) have made it possible to use X-ray absorption spectroscopy to probe changes in electronic configuration and atomic structure as a function of time, beginning from the initial excited state. Both ‘movies’ of coherent or ballistic motion and ‘snaphots’ of local minima or kinetic intermediates are possible. Polarization anisotropy, long exploited in ultrafast optical measurements, permits decomposition of the X-ray transient difference signal into contributions along the direction parallel to the transition dipole initially pumped, and perpendicular to this transition dipole. This decomposition allows the analysis of asymmetric sequential structural changes of photoexcited molecules in isotropic solution. We have used femtosecond X-ray absorption near edge structure (XANES) at the Co K-edge to characterize the excited state dynamics of cobalamins, B12 coenzymes and analogues. The initial subpicosecond dynamics are found to be ballistic rather than kinetic, with sequential structural changes in the plane of the corrin ring followed by expansion of the axial bonds perpendicular to the ring. A change in electronic configuration accompanies the axial bond elongation as the molecule evolves from the initial ‘bright’ state to a ‘dark’ state within a few hundred femtoseconds. Subsequent structural intermediates prior to bond dissociation or ground state recovery are also identified and characterized. The Finite Difference Method Near Edge Structure (FDMNES) program is used to calculate the XANES spectrum for both the ground and excited states. These simulations are used to extract more detailed static and dynamical information from the time- and polarization-resolved XANES difference spectra. **Acknowledgments** This work was supported by the NSF through NSF-CHE 1464584 and NSF-CHE 1836435. Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. In addition, many collaborators at Michigan (A. Deb, K.J. Kubarych, A. Kanishiro, J. Meadows, L. B. Michocki, N. A. Miller, J.E. Penner-Hahn, D. L. Sofferman) the University of Louisville (B. Garbato, M. Toda, P.M. Kozlowski) and at LCLS (R. Alonso-Mori, A. Britz, D. DePonte, J. M. Glownia, J. Koralek, S. Song, T.B. van Driel, D. Zhu) contributed to the success of this work.

Speaker: Prof. Roseanne Sension (University of Michigan)

17:15 Break

8 Tutorial 1 - Keith Nelson (MIT): Transient gratings from optical to x-ray wavelengths

Tuesday, 15 September

9 Tutorial 2 - Giorgio Margaritondo (EPFL) : XFEL basics, generation and applications

Slides Tutorial-Margaritondo2020.pdf

11:00 Break

10 Virtual Tours (FLASH & European XFEL)

13:00 Break

Session 4 - Laser Physics

11 Quantum optics in strong laser fields

I will present the recent achievements on the synthesis of Quantum Optics with Strong field laser physics. Specifically, I will describe how the strong-field laser-matter interaction can lead to the generation of non-classical light-states which carry the information of the ultrafast dynamics of the interaction.

Speaker: Dr Paraskevas Tzallas (FORTH-IESL)

12 Nonlinear Terahertz spectroscopy and coherent control at accelerator based light sources.

Terahertz (THz) frequency range contains wavelengths between 3 mm to 30 µm that corresponds to the energies between 0.4 meV to 40 meV. Spectroscopy in this range plays very important role in an understanding the solid state physics as there are multiple low energy excitations (phonons, magnons, Higgs mode etc.) that determine numerous physical parameters of the matter. Not only to perform THz spectroscopy but to coherently manipulate the phase of matter through these excitations it is required to develop high field narrow band THz radiation sources with very sensitive probing techniques. THz user facility TELBE at Helmholtz-Zentrum Dresden-Rossendorf provides quasi CW SRF accelerator based narrowband THz radiation at high repetition rates. The source is based on superradiant principle that enables high degree of CEP stability, operation in deep THz frequencies tunable between 100 GHz to 3 THz and repetition rates up to MHz range [1]. The bandwidth of radiation is 10-15 % with pulse energies up to 12 µJ (up to 100 µJ as design parameter) that results in few 100 kV/cm peak field strength. Using pulse-resolved technique we can reach few femtosecond synchronization with ultra-fast laser system that is crucial for studying ultrafast dynamics [2]. The listed above parameters outperform state of the art high repetition rates laser based THz sources and are important for observing novel high field THz phenomena. In this contribution we present concept of superradiant THz sources and our recent results on observing novel high field terahertz phenomena making use of such sources [3-5]. References 1. B. Green, S. Kovalev, V. Asgekar et al., “High-field high-repetition-rate sources for the coherent THz control of matter,” Scientific Reports 6, 22256 (2016). 2. S. Kovalev, B. Green, T. Golz et al., “Probing ultra-fast processes with high dynamic range at 4th-generation light sources: Arrival time and intensity binning at unprecedented repetition rates,” Structural Dynamics 4, 024301 (2016). 3. H. A. Hafez, S. Kovalev, J.-C. Deinert et al, “Extremely efficient terahertz high-harmonic generation in graphene by hot Dirac fermions,” Nature 561, 507 (2018). 4. S. Kovalev, R. Dantas, S. Germanskiy et al., “Non-perturbative terahertz high-harmonic generation in the three-dimensional Dirac semimetal Cd3As2”, Nature Communications 11, 2451 (2020) 5. H. Chu, M. Kim, K. Katsumi et al., „Phase-resolved Higgs response in superconducting curpates“, Nature Communications 11, 1793 (2020)

Speaker: Dr Sergey Kovalev (Helmholtz Zentrum Dresden Rossendorf)

15:00 Break

Session 5 - Magnetic & Correlated Materials

13 Direct observation of a transient fluctuation state mediating a picosecond topological phase transition

Topological phases of matter are highly non-trivial configurations of the electronic or spin system, which often result in exotic and previously unimaginable material properties. The topology endows these states with a remarkable stability that often allows them to be realized at room temperature and with lifetimes exceeding decades. A key challenge, however, is the creation of topological phases since long lifetimes usually imply that switching events are rare. Surprisingly, we find that picosecond nucleation of an extended topological phase, comprising a dense array of nanometer-scale magnetic skyrmions, can be induced by a single femtosecond laser pulse. In this talk, I will discuss the nucleation dynamics of this topological phase transition, which we were able to follow in real time during the early user operation of beamline SCS at the European XFEL [1]. Using time-resolved small angle x-ray scattering, we discovered that rapid, homogeneous nucleation of the skyrmion phase is mediated by a previously undisclosed transient fluctuation state. This state, which is characterized by high spatial frequency magnetic fluctuations, persists for approximately 100 ps after exciting our magnetic multilayer with a femtosecond, infrared laser pulse. The topological phase emerges from these fluctuations by nucleation and coalescence, a mechanism that goes well beyond existing theories of topological phase transitions such as the Kibble–Zurek mechanism and the Berezinskii–Kosterlitz–Thouless transition. The process is completed on a time scale of 300 ps. Using atomistic spin dynamics simulations, we confirm that the fluctuation state is key to the ultrafast increase of the global topological charge, enabled by an almost complete elimination of the topological energy barrier in this transient state of matter. Figure 1: Time-trace of the measured magnetization of skyrmions (total intensity) as well as their distance (correlation length) and size (diameter), at a time delay relative to the incident infrared pulse. Inset: Representative real space image of skyrmion state long after the excitation. Scale bar 200 nm. [1] F. Büttner, B. Pfau, M. Böttcher, M. Schneider, G. Mercurio, C. M. Günther, P. Hessing, C. Klose, A. Wittmann, K. Gerlinger, L.-M. Kern, C. Strüber., C. von Korff Schmising, J. Fuchs , D. Engel, A. Churikova, S. Huang, D. Suzuki, I. Lemesh, M. Huang, L. Caretta, D. Weder, J. H. Gaida, M. Möller, T. R. Harvey, S. Zayko, K. Bagschik, R. Carley, L. Mercadier, J. Schlappa, A. Yaroslavtsev, L. Le Guyarder, N. Gerasimova , A. Scherz, C. Deiter, R. Gort, D. Hickin, J. Zhu, M. Turcato, D. Lomidze, F. Erdinger, A. Castoldi, S. Maffessanti, M. Porro, A. Samartsev, J. Sinova, C. Ropers, J. H. Mentink, B. Dupé, G. S. D. Beach, and S. Eisebitt (unpublished).

Speaker: Dr Felix Büttner (Helmholtz-Zentrum Berlin)

14 Coherent structural dynamics of quasi low-dimensional materials

Materials with structural and electronic properties with reduced dimensionality are an important class that displays many interesting properties both in and out of equilibrium. In this talk I report on two recent studies on such systems. First, I describe an investigation of the coherent vibrational dynamics associated with the quasi-1D material K0.3MoO3, a charge density wave material. Here we observe an anomalous damping effect that appears under conditions where the ultrafast symmetry change in the interatomic potential occurs. The results demonstrate a novel means of coherent control of lattice dynamics by manipulating damping. I also report on recent XFEL experiments on the quasi-2D material Td-WTe2 that show clearly the ability to coherently modulate inter- and intra-planar vibrational modes that directly impact both the carrier properties and move the system toward another topological phase. These results indicate a possible new means of constructing ultrafast thin film devices with novel transport properties

Speaker: Steven Johnson (ETH Zürich)

16:05 Break

Session 6 - Material Science

15 Ultrafast electron localization in a correlated metal

Ultrafast electron delocalization induced by a fs laser pulse is a well-known process in which electrons are ejected from the ions within the laser pulse duration.(1) However, the speed of electron localization on an atom by an excitation is unknown. Here, we demonstrate by means of x-ray absorption spectroscopy that an electron localization process into 4f states of a Eu-based correlated metal occurs within few-hundred femtoseconds. Our data suggests that the driving force for this process is either a reduction of the 4f states energy, a change of their bandwidth or an increase of the hybridization between the 4f/3d states. The observed ultrafast electron localization process raises fundamental questions for our understanding of electron correlations and their coupling to the lattice.

Speaker: Urs Staub (PSI)

16 New Light on the Frontier Matter in Extreme Conditions

The study of matter under extreme conditions is a highly interdisciplinary subject with broad applications to materials science, plasma physics, geophysics and astrophysics. Understanding the processes which dictate physical properties in warm dense plasmas and condensed matter, requires studies at the relevant length-scales (e.g., interatomic spacing) and time-scales (e.g., phonon period). Experiments performed at XFEL lightsources across the world, combined with dynamic compression, provide ever-improving spatial- and temporal-fidelity to push the frontier. This talk will cover a very broad range of conditions, intended to present an overview of important recent developments in how we generate extreme environments and then how we characterize and probe matter at extremes conditions– providing an atom-eye view of transformations and the fundamental physics dictating materials properties. Examples of case-studies closely related to Earth and planetary science relevant materials will be discussed.

Speaker: Dr Arianna Gleason (SLAC/Stanford)

17:15 Break

17 FOE prize award and talk (R. Boll)

18 Parallel poster session

Wednesday, 16 September

19 Tutorial 3 - Majed Chergui (EPFL): Novel insights into the electronic and structural dynamics of molecules, proteins and solids from ultrashort X-ray studies

11:00 Break

20 Poster Session

12:00 Break

Session 7 - New Developments

21 From ultrafast lasers to FELs: extending experimental techniques to short wavelengths.

The advent of free-electron-laser (FEL) sources provided pulses of extreme ultraviolet (EUV) and soft to hard X-ray radiation with coherence and time-duration comparable to those of table-top fs-laser sources. In the first place, this led to the extension from the optical to the short-wavelength regime of time-domain techniques, allowing the direct measurement of decay times of fundamental excitations in matter. Now, after one decade of operation, FELs have allowed the development of experiments at the crossroad between typical table-top laser and synchrotron techniques, the evolution of X-ray non-linear optics and the emergence of new experimental approaches. The FERMI facility in Trieste (Italy) is unique in the FELs landscape because of its seeded emission scheme, resulting in superior performances in terms of shot-to-shot pulse stability. Of particular interest for spectroscopic applications are its broad tunability, spectral purity (approaching the Fourier-transform limit), sub-linewidth stability and timing jitter. Transverse and temporal coherence are those expected from a true laser, and have been exploited in a series of pioneering experiments both in non-linear optics and in atomic and molecular physics. In this talk, I will present an experimental scheme for two-dimensional X-ray absorption spectroscopy, which uses FERMI in the optical klystron configuration, thus simulating the stochastic emission of SASE sources. Then, I will show how non-linear optical schemes, and in particular transient grating spectroscopy can be exploited for single shot determination of spectral and temporal characteristics of FEL pulses. Finally, I’ll discuss a novel approach to extend transient grating spectroscopy to the hard X-ray regime, where, up to now, attempts were hampered by shot-to-shot pulse fluctuations.

Speaker: Dr Laura Foglia (Elettra Sincrotrone Trieste)

22 Development of Novel X-ray Optics for Advanced XFEL Applications

Development of X-ray optics is critically important for enabling advanced applications with XFEL sources with ultrabrilliant, ultrafast, coherent properties. In this presentation, I will introduce our latest activities in regarding with this topic, such as split-and-delay optics (SDO) [1], ultimate nano focusing, and reflection self seeding using a micro channel-cut crystal monochromator [2,3], and temporal diagnostics with the Hanbury-Brown and Twiss interferometry [4,5]. Novel X-ray optics for ultralow emittance synchrotron sources will be also discussed.

Speaker: Dr Makina Yabashi (RIKEN SPring-8 Center)

15:00 Break

Session 8 - Non-linear Science & Relaxation Phenomena

23 X-ray optical wavemixing: Theory, challenges and prospects

With the development of optical lasers, nonlinear wavemixing techniques were established that have since found their applications across a broad range of science and technologies. Today, the emergence of FELs allows to transfer such wavemixing concepts towards increasingly higher photon energies and thereby gain new probes of matter beyond the linear response. Of particular interest are processes of parametric x-ray optical wavemixing; these promise imaging capabilities similar to regular x-ray diffraction with additional spectroscopic selectivity tunable via the optical admixture. Intriguingly, this provides a method to specifically address valence electrons. In order to explore the potential of such processes, a theoretical description of parametric x-ray optical wavemixing is presented, which adopts a scattering perspective in non-relativistic QED. Applications in terms of sum- and difference-frequency generation will be discussed with a view to both present and future experiments.

Speaker: Mr Dietrich Krebs (UHH / DESY / MPSP)

24 Lattice dynamics at the nanoscale probed by the extreme ultraviolet transient grating approach

Fully coherent ultrafast pulses in the extreme ultraviolet (EUV) and soft x-ray spectral range are nowadays available at seeded free electron laser (FEL) facilities, such as FERMI (Trieste, Italy). This new generation of light sources has allowed the development of an experimental approach based on the exploitation of the four-wave-mixing response, in the so-called transient grating (TG) scheme [1,2]. EUV TG can be used to probe several types of dynamical processes and, recently, has been implemented with FEL-pump/FEL-probe capabilities at the TIMER beamline [3,4]. This latter development removed the main limitation of EUV-optical TG experiments [1,2], that was the access to a length-scale range shorter than optical wavelengths (< 1 μm). Therefore, TG experiments fully based on EUV pulses are now able to explore the mesoscopic (10’s of nm) range, hardly accessible by other means. In this presentation we report on these new experimental capabilities and on the first experimental projects, carried out in close collaboration with the users. As a result, phonon dynamics down to 24 nm wavelength and thermal responses down to a sub-10 nm thermal transport distances were observed in a number of samples [4-6]. In such a regime we found that the thermal transport process in crystalline samples shows a strongly non-diffusive nature, while in amorphous silicon nitride it is still compatible with regular thermal diffusion [4]. Applications of the EUV TG approach can also be envisioned in other research fields, as for instance in the study of mesoscopic structural relaxations in liquids or nanoscale magnetic dynamics; the latter was demonstrated in recent experiments [6,7]. Finally, it is worth stressing that these data represent unique experimental evidences of time-resolved four-wave-mixing processes exclusively driven by EUV pulses [6,7]. This is a remarkable advance in nonlinear optics and holds great potential for a wider class of nonlinear x-ray experiments, so far conceived only on theoretical grounds. References [1] F. Bencivenga et al., Nature 520, 205 (2015) [2] A.A. Maznev et al., Appl. Phys. Lett. 113, 221905 (2018) [3] L. Foglia et al., Phys. Rev. Lett. 120, 263901 (2018) [4] F. Bencivenga et al., Science Advances 5, eaaw5805 (2019) [5] D. Naumenko*, R. Mincigrucci* et al., ACS Appl. Nano Mater. 2, 5132 (2019) [6] G. Monaco, K.A. Nelson, A.A. Maznev unpublished [7] D. Ksenzov et al., in preparation

Speaker: Filippo Bencivenga (Elettra-Sincrotrone Trieste)

16:10 Break

Session 9 - Attosecond Science

25 Transmitting the change: From watching to steering electron dynamics with intense FEL fields

The advent of Free-Electron Lasers (FELs) has made a dream come true: Imaging and spectroscopy of atomic and molecular structural and electronic dynamics on their natural time scale by XUV/X-ray pump&probe pulses. Besides slowly realizing this dream, FELs keep fueling a "dream-on revolution" by inspiring new questions: Can we understand the vast variety of nonlinear (and complex multi-electron) interactions of XUV/x-ray light with matter? Can we even use the strong FEL fields at high frequencies to steer electron dynamics on an atomic level? Here, we will discuss some of our recent experiments and insights into extreme FEL interactions with matter. A home-built transient-absorption spectroscopy beamline setup brought to FLASH at DESY/Hamburg allowed the observation of characteristic gas-phase atomic and molecular spectra in "Fraunhofer-type" transmission geometry. This approach allows to study the modification of fundamental resonances as a function of intensity and time delay of two coherently split XUV pulses from the same FEL shot. Focusing intense FEL light near 60 eV on the 2s2p double excitation of a cloud of helium atoms, we observed a change of the characteristic Fano line shape. Modeling this interaction allows to identify the mechanism: A strong coupling between the ground and the (doubly) excited state at the onset of a two-electron Rabi cycle allows to exert phase control of the electronic excitation. Control over the phase is a prerequisite for shaping electronic wavefunctions. It can be shown that short FEL pulses (shorter than the lifetime/dynamics of the excited state) are key to this controllability, while longer pulses essentially only ionize the atom. Towards exploring and understanding larger systems, we turn to neon atoms, where XUVpump/ XUV-probe measurements of the absorbance change in neon gas allowed to directly watch the time-resolved buildup of doubly charged ions in the intense XUV field. Moreover, resonances of Ne2+ were observed as a function of intensity near 50 eV photon energy, exhibiting a spectral (Stark) shift on the order of 50 meV for one out of three lines of the 2p-3d multiplet transitions. A 2-fs coherence spike showed up in the nonlinear XUV-optical absorption spectrum near temporal overlap, pointing at the feasibility of gas-phase multidimensional spectroscopy even with self-amplified spontaneous-emission mode (SASE- )FELs. We conclude with first time-resolved experimental results from iodine-containing molecules with site-specific pump&probe pulses resonant on the iodine 4d core resonance. Here, direct tracking of dissociation dynamics through different molecular geometries becomes possible, and is expected to gain massively by future two-color and broad-band XUV probing methods for extracting additional pathways and providing comprehensive control knobs even for steering chemistry on the fundamental level of atomic-site-selected electrons.

Speaker: Pfeifer Thomas (MPIK Heidelberg)

26 X-ray diffraction imaging with intense sub-fs X-ray puleses

The adventvent of X-ray Free Electron Lasers (FELs) opens the door for unprecedented studies on non-crystallin nanoparticles with high spatial and temporal resolutions [1]. In the recent past, ultrafast X-ray imaging studies have elucidated hidden processes in individual fragile specimens, which are inaccessible with conventional imaging techniques. Examples include airborne soot particle formation [2], metastable states in the synthesis of metal nonaparticles [3], transient vortexes in superfluid quantum systems [4] and non-equilibrium dynamics of laser-superheated nanoparticles [5]. Theoretically, ultrafast coherent diffraction X-ray imaging (CDI) might achieve sub-nanometer resolution in combination with sub-femtosecond temporal precision. The best demonstrated spatial resolution of ultrafast X-ray CDI is still far away from the diffraction limit due to restrictions in X-ray scattering signal strength. The brightness of the diffraction patterns is determined by a combination of several factors such as X-ray photon flux, image imperfections and ultimately, sample damage [6,7]. Ionization through the FEL pulse during the exposure is generally considered detrimental to the quality of single shot images. In general, ionization can lead to increased transparency of the sample and even modify the structure of the sample during the imaging process. In contrast, our study conducted at the Linac Coherent Light Source (LCLS) indicates that ionization might increase the brightness of the diffraction image prior to significant structural damage if the pulses are very short. In our experiment we observe that the X-ray coherent scattering cross section of Xe nanoparticles increases significantly above the 3d absorption edge for X-ray pulses as short as 0.5 - 10 fs. A Monte-Carlo-simulation [8] attributes the observed increase to transient ionic resonances, which are created through X-ray absorption and subsequent electronic damage during the FEL exposure. If the FEL pulses are shorter than few femtoseconds, the electronic damage does not affect the ionic structure. Using the novel XLEAP mode at LCLS [9], we have demonstrated that it is possible to image individual nanoparticles with few nanometer and sub-fs temporal resolutions. Our model also suggests that transient ionic resonances can be exploited with intense sub-fs short pulses even for lighter elements. Overall transient resonances provide a novel avenue in improving the quality of images of in CDI. [1] Neutze, Richard, et al. Nature 406.6797 (2000): 752-757 [2] Loh, N. D. et al. Nature 486, 513–517 (2012). [3] Barke, I. et al. Nat. Commun. 6, (2015):6187. [4] Gomez, L. F. et al. Science 345, 906–909 (2014). [5] Gorkhover,T. et al. Nat. Phot. 10, (2016):93. [6] Aquila, Andrew, et al. Structur. Dyn. 2.4 (2015).: 041701 [7] Ho, Phay J., et al. Nat. Commun 11 (2020). [8] Ho, Phay J., et al. Phys. Rev A 94, no. 6 (2016): 063823. [9] Duris, Joseph, et al. Nat. Phot. 14.1 (2020): 30-36.

Speaker: Dr Taisia Gorkhover (University of Hamburg)

27 Closing remarks