Welcome and Introduction (Presentations from the SMART-X members)

• Michael Odelius (Stockholm University)
• Caterina Vozzi
• Markus Kowalewski (Stockholm University)

A high-repetition rate attosecond light source for time-resolved coincidence spectroscopy (Invited Talks)

• Cord Arnold (Lund University)

The advent of attosecond pulses, produced through high-order harmonic generation (HHG) in the beginning of the millennium, has opened up for time domain spectroscopic approaches with unprecedented, attosecond, time resolution. However, typical attosecond sources are often impaired by the low repetition rate of the driving lasers, potentially resulting in measurements with insufficient statistics and low signal-to-noise ratio. This is particularly true for experiments where several reaction products must be detected in coincidence and for applications where space charge effects compromise spatial and spectral resolution. In this talk, I will discuss the development of a high-repetition rate (200 kHz) attosecond source and its first applications in attosecond pump-probe interferometry. Key to our approach is the combination of a well-controlled, high repetition rate, CEP-stable, few-cycle laser source with a specially designed gas target for efficient harmonic generation and a homemade 3D coincidence photoelectron/ion momentum detector. While the most established techniques in the field of attosecond science either use single attosecond pulses or long trains of pulses, we explore the hitherto mostly overlooked intermediate regime of short trains, i.e. with only a few pulses. We present a proof-of-principle measurement of one-photon double-ionization in He with full angular resolution, achieving detection rates comparable to synchrotron facilities. We have set the conditions for future studies of highly correlated many-body processes in the temporal domain.

Probing elementary charge transfer processes in the condensed matter by femtosecond soft X-ray spectroscopy (Presentations from the SMART-X members)

• Erik T. J. Nibbering (Max Born Institute)

Charge transfer processes are ubiquitous in nature. They are responsible for energy transport and signal transduction in biomolecular systems. Charge carrier dynamics play also key roles in light harvesting systems used for solar cell technologies and hydrogen fuel cells. Probing the transient local charge densities with high spatio-temporal resolution is a prerequisite for a better understanding of ultrafast redox reactions that are at play in these systems. In the past century, X-ray spectroscopy has been a method of choice to probe the local electronic structure of matter. With the advent of ultrashort soft X-ray sources based on Free Electron Lasers and High-order harmonic generation using table-top femtosecond lasers, time-resolved measurements of the elementary steps occurring in charge transfer processes has now become feasible. In our group, we aim to investigate ultrafast charge transfer processes occurring in donor-acceptor molecular systems in solution and the carrier dynamics in metal-oxide semiconductors by time-resolved soft X-ray spectroscopy. We will present the high harmonic source that we have recently developed providing femtosecond soft X-ray pulses in the water window spectral range. The source was first used to characterize the soft X-ray absorption spectra of simple molecules and ions in aqueous solution and of Fe$_2$O$_3$ thin materials. We will present as well our first time-resolved investigation of the ionization dynamics of gas-phase molecules using the newly developed source. Our results pave the way towards the investigation of femtosecond charge flow in solutes and solids.

Energy Conversion Pathways in Graphite from Attosecond Soft X-ray Spectroscopy (Presentations from the SMART-X members)

• Jens Biegert (ICFO)

The conversion of energy, between photons, charge carriers, and the lattice is of fundamental importance to advance materials and devices, yet such insight remains incomplete due to experimental challenges in disentangling the various effects on overlapping time scales. Here, we show that attosecond core-level X-ray spectroscopy can identify these mechanisms with attosecond precision and across a picosecond range. We apply this methodology to graphite since its investigation is complicated by a variety of mechanisms occurring across a wide range of temporal scales. Our investigation reveals, through the simultaneous real-time measurement of electrons and holes, the different dephasing mechanisms for each carrier type dependent on excitation with few-cycle-duration light fields. We observe coherent excitation of the non-Raman active A1' phonon simultaneously with the Raman-active E2g phonon within only 20 fs by the incoherent electron-phonon scattering. Further, the measurement identifies the A1' phonon as the dominant channel for electronic de-excitation. These results demonstrate the general ability of our methodology to distinguish the various dynamic contributions to the flow of energy inside materials on their native time scales.

Attosecond spectroscopy of semiconductors

• Caterina Vozzi (CNR-IFN)

The aim of this talk is to present the activity of CNR-IFN and the Physics Department of Politecnico di Milano on ultrafast spectroscopy within the SMART-X network. Ultrafast X-ray spectroscopy allows the study of light-matter interaction with unprecedented temporal and spatial resolution with the further advantages of being element-selective and oxidation- and spin-state specific. The investigation of the properties of core electrons at ultrafast time scales promises to enlighten the dynamics occurring in complex materials. I will discuss the recent developments we implemented toward attosecond X-ray spectroscopy based on tabletop sources. In particular, I will present the development of the transient absorption/reflectivity beamline for the study of materials based on efficient high-order harmonic generation in fused-silica chips fabricated by femtosecond laser micromachining. I will also present recent results on HHG spectroscopy from germanium crystals.

Simulating linear and non-linear time resolved spectra - from gas phase molecules to solution

• Markus Kowalewski (Stockholm University)
• Michael Odelius (Stockholm University)

We will present an overview over our theory projects that involve the modelling of molecular dynamics and the simulation of ultra-fast X-ray spectra. In the first part we will give an overview over potential spectroscopic methods, which can be used to identify conical intersections and non-adiabatic dynamics in the excited state dynamics of molecules. We will present a theoretical study on ultra-fast X-Ray methods, which make use of ultrashort soft X-Ray laser pulses and compare the results for different probes to detect a conical intersection in pyrrole as an example molecule. In the second part, the challenges for quantum chemistry in the modelling of excited state dynamics in solution and associated transient X-ray signals will be discussed. Interactions and hybridization with the surrounding solvent environment strongly influence both dynamics and X-ray spectra, and need to be taken into account even when a solute is examined with local X-ray probes. On the other hand, an accurate description of valence- and core-excited states might require advanced quantum chemistry calculations, which are difficult to handle for large systems.

QUASI-2D V. D. WAALS METALLIC FERROMAGNETS (Presentations from the SMART-X members)

• Athanasios Dimoulas (NCSR DEMOKRITOS)

Van der Waals metallic ferromagnets (e.g. CrxTey, Fe3GeTe2) with a Curie temperature near 300 K are a new addition to the family of 2D materials. They possess perpendicular magnetic anisotropy down to a few monolayer regime and are reported to host skyrmions. Their combination with spin orbit torque materials (e.g. topological insulators (TI)) may lead to efficient charge to spin conversion for spintronic devices. Apart from technology, there is great scientific interest to correlate the observed large anomalous Hall effect (AHE) with the electronic bandstructure and the possible non-trivial topology of these materials. The broken time reversal symmetry in magnetic materials generates a large Berry curvature (BC) at (near) energy degeneracy points where the metallic bands cross. The latter quantity acts as an effective magnetic field in reciprocal space producing anomalous Hall currents without external magnetic field. We will first present our DFT band structure calculations for the and Cr2Te3, Fe3GeTe2. Then, we will describe the wafer scale epitaxial growth [1,2] of the 2D ferromagnets/Bi2Te3 TI [3] combinations on different substrates by MBE and we will show evidence of their structural quality by in-situ STM, RHEED and ex-situ Raman and XRD. The band structure is imaged by in-situ ARPES and compared with theoretical calculations. Based on magnetic hysteresis measurements (SQUID, AHE), the optimum growth temperature, layer thickness and substrate are identified for spin torque and efficient charge to spin conversion. The measured AHE is correlated with DFT calculations of anomalous Hall conductivity originating from sizeable BC in these materials. Acknowledgements: H2020 FET PROAC project SKYTOP; H2020 MSCA ITN project SMART-X References [1] H. Li et al., ACS Appl. Nanomaterials DOI: 101021/acsnm 9b01179 (2019) [2] H. Wang et al., ACS Nano 14, 10045 (2020) [3] S. Fragkos et al., Phys. Rev. Materials 5, 014203 (2021)

Time-resolved soft x-ray absorption and resonant inelastic x-ray scattering of transition metal oxides (Presentations from the SMART-X members)

• Frank de Groot (Debye Institute of Nanomaterials Science Utrecht University, Netherlands)

Soft x-ray absorption and resonant inelastic x-ray scattering are briefly introduced [1]. In 2p3d resonant inelastic x-ray scattering (RIXS) one scans through the 2p XAS edge and measures the low energy excitations, including phonons, magnons, dd-excitations and charge transfer [2].Recently we measured time-resolved XAS and RIXS using the PAL-XFEL source in Pohang [3,4]. The oxygen K edge and metal L edges of CuWO$_4$ and Fe$_2$O$_3$ have been measured in relation to their application as photo(electro)catalyst systems. References: [1] "Core Level Spectroscopy of Solids" Frank de Groot and Akio Kotani (Taylor & Francis CRC press, 2008) [2] Nag et al. Phys. Rev. Lett. 124, 067202 (2020) [3] Ismail et al. Phys. Chem. Chem. Phys. 22, 2685 (2020) [4] Uemura et al. J. Phys. Chem. C. (2021)

Understanding Defect Physics to Stabilize Metal-halide Perovskite Semiconductors for Optoelectronic Applications

• Annamaria Petrozza

Semiconducting metal-halide perovskites present various types of chemical interactions which give them a characteristic fluctuating structure sensitive to the operating conditions of the device, to which they adjust. This makes the control of structure-properties relationship, especially at interfaces where the device realizes its function, the crucial step in order to control devices operation. In particular, given their simple processability at relatively low temperature, one can expect an intrinsic level of structural/chemical disorder of the semiconductor which results in the formation of defects. Here, first I will summarize our understanding of the nature of defects and their photo-chemistry, which leverages on the cooperative action of density functional theory investigations and accurate experimental design. Then, I will show the correlation between the nature of defects and the observed semiconductor instabilities. Instabilities are manifested as light-induced ion migration and segregation, eventually leading to material degradation under prolonged exposure to light. Understanding, controlling and eventually blocking such material instabilities are fundamental steps towards large scale exploitation of perovskite in optoelectronic devices.

Light-matter interactions: from ab initio molecular dynamics and machine learning to x-ray spectroscopy (Invited Talks)

• Morgane Vacher (Université de Nantes, CNRS,  CEISAM UMR 6230)

Computer simulations are a key complement to experiments in the laboratory, providing much greater details of a molecular process than can be observed experimentally. For instance, ab initio molecular dynamics simulations are often key to the understanding of the mechanism, rate and yield of chemical reactions. One current challenge is the in-depth analysis of the large amount of data produced by the simulations, in order to produce valuable insight and general trends. In the first part of my talk, I will present recent machine learning analysis tools used to extract relevant information from ab initio molecular dynamics simulations without a priori knowledge on chemical reactions. It is demonstrated that, in order to make accurate predictions, the models evidence empirical rules that are, today, part of the common chemical knowledge. This opens the way for conceptual breakthroughs in chemistry where machine analysis would provide a source of inspiration to humans. In the second part of my talk, I will show recent experimental and theoretical results on the photo-induced dynamics of an iron photosensitizer. Coherent structural dynamics in the excited state of an iron photosensitizer was observed through oscillations in the intensity of Kalpha x-ray emission spectroscopy (XES). Using multiconfigurational wavefunction calculations, we explain the origin of the unexpected sensitivity of core-to-core transitions to structural dynamics.

Dynamics in core-excited molecules with HAXPES and MOSARIX spectrometers (Presentations from the SMART-X members)

• Iyas ISMAIL (CNRS-Sorbonne Université)
• Tatiana MARCHENKO (CNRS-Sorbonne Université)

Along with the rapid development of the laser technology allowing for time-resolved studies, a remarkable progress in the synchrotron technology provides nowadays a wealth of information on the relaxation mechanisms of core-exited systems through high energy-resolution electron and X-ray spectroscopy and essentially complements the time-resolved studies [1]. We use X-ray emission spectroscopy and Auger electron spectroscopy for addressing ultrafast dynamic processes in deeply core-excited molecules in the tender X-ray range (photon energy of 1-12 keV). In the first part, we will present an example demonstrating how the information on the dynamic response of molecules can be obtained using high-resolution Auger electron spectroscopy. Using hard X-ray photoelectron spectrometer HAXPES installed at the GALAXIES beamline of SOLEIL synchrotron we have demonstrated an evidence of ultrafast proton motion occurring in the double-core-hole states in gas-phase water molecules ionized with hard x-ray radiation [2]. In the second part, progress in the development of a novel spectrometer MOSARIX in the tender x-ray range will be presented [3]. The aim is to achieve a capability for high-resolution and high-efficiency measurements using multi-mosaic crystals in Von Hamos geometry. Furthermore, MOSARIX will be equipped with a time- and position-sensitive detection allowing extension towards time-resolved X-ray emission spectroscopy in coincidence with ion or electron spectroscopy at pulsed X-ray sources. [1] M. N. Piancastelli et al., J. Phys. B: At. Mol Opt. Phys. 50, 042001 (2017) [2] T. Marchenko et al., Phys. Rev. A 98, 063403 (2018) [3] I. Ismail et al., submitted

Bringing the chemistry at the FERMI free electron laser: recent advances and capabilities (Presentations from the SMART-X members)

• Riccardo Mincigrucci (Elettra Sincrotrone trieste)

FERMI is a world unique free electron laser, that thanks to the seeding scheme can deliver coherent pulses up to the carbon K edge, with an exquisite polarization, photon energy and timing control. These properties, corroborated by multi-colors emission schemes permit to perform non-linear optics time-resolved experiments, with the long dreamed achievement of the chemical sensitivity. In this talk, I will present recent non-linear optics experiments conducted at the EIS-TIMER and EIS-TIMEX beamlines aimed at resolving with chemical sensitivity different dynamics in prototypical systems. In particular, I will focus on the vibrational dynamic of Ibuprofen and Hydroxychloroquine projected on their atomic constituents and a proof of principle study performed on the thermite compound designed to visualize the temporal evolution of the redox reaction happening between iron oxide and aluminum. These preliminary studies, combined with the transient grating capabilities of the EIS-TIMER beamline, open the way to the design of more complex experiments aimed at elucidating the role of coherent vibrations on the charge transfer mechanisms.

X-ray optical spectroscopies form first principles (Presentations from the SMART-X members)

• Umberto De Giovannini (MPSD)

Transient X-ray near-edge absorption spectroscopy (XANES) and high harmonic generation (HHG) are powerful techniques to study structural and dynamical properties of materials. From the first-principles modelling perspective they present several challenges such as accessing core electrons, capturing correlations and excitons and describing the coupling to phonons. Overcoming these challenges for/with TDDFT and being able to unveil the physics behind experiments in this area will be the main focus of the project that we will develop within the Smart-X network.

Supervisory Board Meeting (Supervisory Board Meeting)

Imaging ultrafast dynamics in isolated nanoparticles (Invited Talks)

• Daniela Rupp (ETH Zürich, Switzerland)

With the extremely intense short-wavelength pulses of free-electron lasers (FELs) and high harmonic sources (HHG), novel experiments with highest spatial and temporal resolution become possible. One key example is coherent diffraction imaging of individual nanoparticles. Here, the elastically scattered photons form an interference pattern, which encodes structural information in a 'snapshot'. This enables the in-situ study of fragile structures, for example of superfluid helium nanodroplets, and time-resolved investigations of ultrafast dynamics such as laser induced ultrafast melting in metal nanoparticles. Even changes in the electronic structure due to excitation and ionization alter the scattering response and can therefore be followed with diffraction imaging. Because electron dynamics occur on a timescale shorter than the typical tens of femtosecond pulse durations, we expect an exciting development from the current progress at X-ray FELs (e.g. at Swiss-FEL) and HHG sources towards high-intensity attosecond pulses. In my talk I will present time-resolved diffraction experiments on silver clusters and helium nanodroplets using extreme ultraviolet (XUV) pulses. The comparably long wavelengths allow for the measurement of wide-angle diffraction patterns that contain three-dimensional information, thus enabling the structural characterization of superfluid spinning droplets and silver nanopolyhedra. I will also discuss time-resolved pump-probe experiments using optical laser pulses with moderate intensities to excite the isolated nanoparticles. We observe interesting switching dynamics in the ultrafast electronic response of helium nanodroplets and find indications of ultrafast melting and instable phase explosions in superheated silver clusters. These results demonstrate the capability of diffraction imaging to visualize ultrafast nanoscale dynamics in highly excited matter.

Approaches towards a novel sCMOS camera platform for high repetition rate X-ray imaging and single photon detection (Presentations from the SMART-X members)

• Conrad Friedrich (greateyes GmbH)

Detectors suitable for high repetition rate imaging and spectroscopy as well as for single photon detection are a key prerequisite for high sensitivity, transient EUV & soft X-ray absorption experiments. For this purpose existing CCD-based technologies first were compared to a new approach based on novel XUV/X-ray sensitive (scientific) CMOS sensors which recently have become commercially available. To allow for the detection of single photons at lower energies it is necessary to extend the photon shot-noise-limited regime of the detector towards the lowest possible levels, in particular for those events generating only very few or even single electron-hole pairs. Therefore an existing camera hardware platform currently is revised and optimized for the integration of a variety of new CMOS-type sensors. An overview of the progress and current road-map of the ongoing hardware redesign is given, including readout electronics, cooling system and overall mechanical design concepts. The envisaged performance characteristics of the camera as well as the planned key firmware and software functionalities are presented and an outlook of the planned final characterization steps of the camera (e.g. efficiency, linearity, sub-pixel resolution, pile-up behaviour) is given.

Shaping ultrafast pulses with liquid crystals (Presentations from the SMART-X members)

• Aurélie Jullien (Institute de Physique de Nice, UCA, CNRS)

In the framework of a joint laboratory between INPHYNI and FASTLITE, we disclose the use of thick nematic liquid crystals to the temporal shaping of ultrashort pulses. We first propose to exploit the large and electrically controllable birefringence of nematic liquid crystals for precise control of the group and phase delays of femtosecond pulses, which is realized by appropriately addressing one or two electrically-driven cells. We then present an innovative thermo-optically addressed liquid crystal device, enabling continuous spectral phase shaping over a spectral bandwidth spanning from 540nm to 2500nm (450Thz). The modulation dynamic is large enough to shape single-cycle pulses.

ESR Round table discussion (ESR Round table discussion)

OSA Chapters

• Yann Amouroux (OSA Europe)

Group discussion and gentle introduction to how OSA Chapters operates and the benefits involved in taking part as an individual. This will be an informal conversation, encouraging participants to ask questions in this first meeting with OSA.

Concluding remarks

• Caterina Vozzi (CNR-IFN)
• Michael Odelius (Stockholm University)
• Markus Kowalewski (Stockholm University)