SMART-X Stockholm Network Symposium

7 Apr 2021, 09:00 → 9 Apr 2021, 17:00 Europe/Stockholm

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

Welcome to the first network Symposium of the SMART-X Innovative Training Network. The Symposium will be held virtually on Zoom from April 7-9 2021 in Stockholm. 

You can find the Zoom link under Videoconference Rooms below in the left sidepanel.

Wednesday 7 April

09:00 → 09:10 Welcome and Introduction

Speakers: Caterina Vozzi, Markus Kowalewski (Stockholm University), Michael Odelius (Stockholm University)

09:10 → 09:55 A high-repetition rate attosecond light source for time-resolved coincidence spectroscopy

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.

Speaker: Cord Arnold (Lund University)

09:55 → 10:25 Probing elementary charge transfer processes in the condensed matter by femtosecond soft X-ray spectroscopy

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.

Speaker: Erik T. J. Nibbering (Max Born Institute)

10:25 → 10:55 Coffee Break

10:55 → 11:25 Energy Conversion Pathways in Graphite from Attosecond Soft X-ray Spectroscopy

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.

Speaker: Jens Biegert (ICFO)

11:25 → 11:55 Attosecond spectroscopy of semiconductors

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.

Speaker: Caterina Vozzi (CNR-IFN)

11:55 → 12:25 Simulating linear and non-linear time resolved spectra - from gas phase molecules to solution

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.

Speakers: Michael Odelius (Stockholm University), Markus Kowalewski (Stockholm University)

12:25 → 13:30 Lunch Break

13:30 → 14:00 QUASI-2D V. D. WAALS METALLIC FERROMAGNETS

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)

Speaker: Athanasios Dimoulas (NCSR DEMOKRITOS)

14:00 → 14:30 Time-resolved soft x-ray absorption and resonant inelastic x-ray scattering of transition metal oxides

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)

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

14:30 → 15:00 Understanding Defect Physics to Stabilize Metal-halide Perovskite Semiconductors for Optoelectronic Applications

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.

Speaker: Dr Annamaria Petrozza

Thursday 8 April

09:00 → 09:45 Light-matter interactions: from ab initio molecular dynamics and machine learning to x-ray spectroscopy

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.

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

09:45 → 10:15 Dynamics in core-excited molecules with HAXPES and MOSARIX spectrometers

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

Speakers: Dr Tatiana MARCHENKO (CNRS-Sorbonne Université), Dr Iyas ISMAIL (CNRS-Sorbonne Université)

10:15 → 10:45 Coffee Break

10:45 → 11:15 Bringing the chemistry at the FERMI free electron laser: recent advances and capabilities

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.

Speaker: Riccardo Mincigrucci (Elettra Sincrotrone trieste)

11:15 → 11:45 X-ray optical spectroscopies form first principles

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.

Speaker: Umberto De Giovannini (MPSD)

11:45 → 13:00 Lunch Break

13:00 → 14:30 ESR Poster Session: Parallel Sessions

Individual ESR Posters
Secondment hosts are encouraged to visit their guests

13:00 CEP stability in high power OPCPAs

Optical Parametric Chirped-Pulse Amplifiers (OPCPAs) are tunable femtosecond sources widely employed for spectroscopy and high-field physics, for instance to generate attosecond pulses. A key property of such sources is the ability to produce, to some extent, pulses with a stable shot-to-shot Carrier Envelope Phase (CEP). This property is mandatory for most of the experiments involving highly nonlinear light-matter interaction.
However, the stability of the CEP is harder to achieve for high-energy, high average power, complex systems. One suspected cause of remaining CEP instability is the nonlinear stage for supercontinuum generation (SCG). In addition, SCG is used both in the OPCPA architecture itself and as a part of the metrology to evaluate CEP stability.
The PhD project thus focuses on the quantitative study of CEP noise sources, including SCG. This poster/presentation shows the proposed interferometric experimental setup to measure the phase stability during SCG process and assesses the influence of the most key parameters on said stability.

Speaker: Benjamin Maingot

13:00 EVAPORATION AND PHOTO-STRUCTURAL CHARACTERIZATION OF METAL-HALIDE PEROVSKITE SEMICONDUCTORS

Perovskites have become a popular material system for fabricating photovoltaics and various optoelectronic devices. Different variations of solution-processing by spin-coating have been investigated over the years. However, perovskites can also be deposited by thermal evaporation. This technique offers the unique possibility to accurately control the deposition of smooth and uniform multilayers of perovskite materials, without the need for thermal annealing and avoiding the use of toxic solvents. E-beam evaporation is a subclass of thermal evaporation that enables to control more accurately the film growth thanks to a local heating of the precursors and a reduced deposition rate. Films produced in this way will be adequate for characterization through photo-emission electron microscopy and X-ray ultrafast spectroscopy. The goal is the fabrication of thin, uniform and smooth PSCs of different dimensionalities and chemical compositions. The e-beam setup optimization is currently ongoing using as prototype material all inorganic CsPbI3. This composition is particularly suitable because only the correct stoichiometry allows the growth of the film in the perovskite phase at room temperature.

Fabricated samples will be deeply characterized to validate their quality. Their physics will be elucidated as a function of their chemical and structural characteristics, as well as thin-film processing. For this reason, in parallel to the optimization of the vapor deposition method, solution-processed perovskite FaCsSnI3 is investigated. The optical quality of the samples is validated by the characterization of the emissive properties of the thin film i.e. photoluminescence quantum yield and transient dynamics. The surface characterization will be based on photo-emission electron microscopy (Time-resolved PEEM) to clarify the peculiar physic of defects and structural-property relationship exploited in the relation among molecular interactions, structural deformations, and optoelectronic properties.

Speaker: Antonella Treglia (IIT)

13:00 Femtosecond XAS and RIXS Unravel the Electronic Structure of CuWO4 and Fe2O3 Based Photoanodes for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is an auspicious approach to convert solar energy directly into fuel. Hitherto, TiO$_2$, WO$_3$, and α-Fe$_2$O$_3$ have been the most studied metal oxides as photoelectrodes. However, their large bandgap and extremely short charge carrier diffusion lengths are hurdles to attain higher solar-to-hydrogen conversion efficiencies. Recently, copper tungstate (CuWO$_4$) has garnered significant attention owing to its relatively smaller bandgap and improved charge mobility characteristics [1,2]. Nonetheless, only a few seminal studies have been reported to understanding the fundamental electronic structure of CuWO$_4$ [3]. Femtosecond (fs) XAS has flourished as a potent technique that concurrently reveals both electronic and atomic structures and potentially sheds light on their nonequilibrium dynamic interplay, which has a preponderant role in most of the ultrafast mechanisms [4,5]. Besides, fs-RIXS delves into ultrafast electronic structure changes in an element- and site-specific manner at intrinsic fs-time scales of photoinduced dynamics [4]. We intend to deploy both table-top fs soft x-ray sources (for 3p XAS of Fe and Cu) and compare their results with RIXS performed at XFELs and static RIXS performed at synchrotrons, both with hard x-rays for the Cu 1s XAS/RIXS and tungsten 2p XAS/RIXS at SACLA/European XFEL and soft x-rays for the Cu 2p XAS/RIXS at Pohang XFEL. In addition, we will perform RIXS calculations based on first-principle calculations coupled to multiplet calculations for the excited state and RIXS experiments.

References:
[1] J. E. Yourey, B. M. Bartlett, J. Mater. Chem. 2011, 21, 7651.
[2] C. R. Lhermitte, B. M. Bartlett, Acc. Chem. Res. 2016, 49, 1121.
[3] C. M. Tian et al., J. Mater. Chem. A 2019, 7, 11895.
[4] A. S. M. Ismail et al., Phys. Chem. Chem. Phys. 2020, 22, 2685.
[5] Y. Uemura et al., J. Phys. Chem. C. 2021. (https://doi.org/10.1021/acs.jpcc.0c10525)

Speaker: Masoud Lazemi (Utrecht University)

13:00 Investigation of ultrafast charge transfer in push-pull chromophores using table-top soft x-ray HHG spectroscopy

Pull-push chromophores have the donor acceptor structure where it promotes electrons transfer from electron-rich donor group to electron-deficient acceptor group. It can be linked together by chains of different structures and length such as π-conjugated polyene, which are prototypical systems for intramolecular electron transfer processes. Understanding the dynamics of ultrafast charge transfer processes in donor-acceptor molecular systems generally requires a femtosecond temporal resolution to identify the vibrational coupling modes responsible for the electron transfer and to directly visualize the electronic motions. This project aims to develop and improve the ultra-thin liquid flat-jet and a table-top source of soft X-ray pulses providing photons in the water window (280-540 eV) spectral range, and to perform transient soft X-ray absorption experiments to unravel charge transfer processes in simple acceptor-donor molecular systems in solution.

Speaker: zhuangyan zhang

13:00 Measuring strong field Valleytronics in h-BN

Promotion of carriers to different points in k-space provides the basis for a new family of devices, where the excitation between specific valley states can be used as a switch to perform logic operations, exceeding the limits of conventional electronics for novel technologies.
With advances in stability and control of ultrashort laser pulses, several all-optical approaches to induce valley polarization in materials have been proposed, but still await confirmation through experiments.
One recent proposal using light induced states in hexagonal-BN gained a lot of attention as the mechanism is wavelength independent making it highly versatile.
In this approach a trefoil shaped field is used as the pump, making it fit to the crystal symmetry of h-BN. By rotating this triangular electric field, it is possible to match the boron or nitrogen atoms in the hexagonal lattice of the sample and switch between excitation into either the K or K' valley.

Building such a tailored pump is the first challenge in realizing this method experimentally. In this work we use our 160kHz 3$\mu$m-OPCPA system and build a Mach-Zehnder interferometer like arrangement. This way we can manipulate the polarizations, handedness, intensity-ratio and delay of the 3$ \mu$m beam and its second harmonic in separate arms and shape the resulting bi-circular pump into the trefoil.

To probe the pumped sample a weak 800nm pulse is used to drive low order harmonics inside the h-BN. Valley polarization is imprinted in the helicity of the harmonics and can therefore be measured with an ellipsometry setup.

Speaker: Igor Tyulnev (ICFO)

Valley_SmartX2021_Keynote2pdf.pdf

13:00 Modelling Advanced Molecular X-Ray Spectra

The process of High-Harmonic Generation (HHG) emits a sequence of short bursts of radiation, which are coherently driven by the generation laser, where emission events occur during each laser half cycle. Each of these short bursts is in the attosecond regime and their interference leads to the observation of odd harmonics. These attosecond pulse trains (APTs), unlike isolated pulses, are readily available in a HHG setup. The goal of this annual research project is the development and simulation of advanced molecular x-ray spectra, such as Transient redistribution of ultrafast electronic coherences in attosecond Raman signals (TRUECARS), employing such sequences of attosecond pulses as pump-probes. In the TRUECARS technique:
$ S(\omega , T) = 2\Im \left\{ \int _{-\infty} ^{+\infty} dt \, e^{i\omega(t-T)}\varepsilon _0 ^* (\omega) \varepsilon_1 (t-T) \langle \psi (t) | \hat{\alpha} | \psi(t) \rangle \right\} $
an hybrid pulse- a combination of a narrow-band (single attosecond, $\varepsilon _0$) and a broadband (femtosecond, $\varepsilon _1$) pulse- is normally used.
After the simulation on model systems, good molecular candidates will be identified to perform quantum chemistry calculations on.

Speaker: Lorenzo Restaino (Stockholm University)

13:00 Post-collision interaction effects in Sulfur KLL Auger spectra of thiophene and thiophene-based polymer P3HT.

Electronic correlations in inner-shell ionized molecules can be probed via Auger spectroscopy. After a high-energy photon ionizes the atomic core shell close to ionization threshold, a slow photoelectron leaves the atom, not without interacting with the ionic field. Decay may take place via emission of an Auger electron, with which the photoelectron may interact. This Coulombic interaction between the three parties is known as post-collision interaction (PCI). From a classical perspective, PCI occurs due to an energy exchange between the ejected Auger electron, photoelectron and the remaining ion. PCI visibly affects photoelectron and Auger spectra in two characteristic ways: the energy distribution represented by the lineshape, becomes distorted, and the position of the lines in the energy spectrum may shift considerably $^1$.

PCI has been extensively studied in gaseous systems, such as argon $^2$ and carbonyl sulfide (OCS) $^3$. However, little is known on manifestation of PCI in solid systems. Using the HAXPES experimental end-station installed at the GALAXIES beamline of the SOLEIL French synchrotron facility $^4$, we measured high-resolution KLL Auger spectra at photon energies between 0 and 200 eV above the sulfur 1s ionization threshold of an aromatic thiophene molecule in the gas phase, and the thiophene-based π-conjugated polymer poly(3-hexylthiophene-2,5-diyl), known as P3HT, prepared as a film.

A stronger PCI shift was observed in P3HT compared to thiophene, which can be tentatively explained by the screening of the photoelectron field in P3HT, considering the dielectric constant of the polymer and the photoelectron retardation within the polymer.

$^1$ Kuchiev et al. https://doi.org/10.1070/PU1989v032n07ABEH002731
$^2$ Guillemin et al. http://dx.doi.org/10.1103/PhysRevA.92.012503
$^3$ Bomme et al. https://doi.org/10.1088/0953-4075/46/21/215101
$^4$ Céolin et al. http://dx.doi.org/10.1016/j.elspec.2013.01.006

Speaker: Nicolas Velasquez (Sorbonne Universite)

13:00 Real-time observation of chemical reactions

Pump-Probe experiments have become routinely employed tools in the field of spectroscopy, because they give the possibility to reach the time-resolution necessary to unravel dynamics of chemical reactions and physical processes. The goal of my PhD project will be that of investigating such kind of phenomena taking advantage of the radiation provided by FERMI, the free electron laser located in Trieste.Thanks to the seeding scheme, FERMI extreme ultra-violet source (EUV) possess the unique capabilities of being finely tuneable, and most importantly, to be jitter-free. The latter feature is of paramount importance to perform time resolved experiments. The combination of these peculiar characteristics allow to perform time-resolved investigations with a chemical selectivity.I will present some preliminary results from an experiment conducted on the thermite reaction aiming at the real-time observation of the redox reaction that transforms Al and Fe2O3 into Al2O3 and Fe. I will then present another possible study on which I might work, which involves the investigation of the influence of selective coherent acoustical phononic excitation on photoactivated charge-transfer processes.

Speaker: ETTORE PALTANIN

13:00 Simulation of Attosecond Transient absorption spectroscopy in Hexagonal Boron Nitride (h-BN)

Two-dimensional (2D) Hexagonal Boron Nitride (h-BN) is an insulator that has a polar covalent B-N bond. Both monolayer and bilayer (h-BN) emerge as an optoelectronic material, which can be used as photodetectors and for photocatalysis. Here in, we performed calculation for Transient absorption spectroscopy (TAS) for monolayer (h-BN) using a low intensity laser pulse, modelling the dynamics with Time dependent density functional theory (TDDFT) numerical pump probe simulation as implemented in the octopus code. The pump probe measurement is numerically simulated by solving the Time dependent Kohn-Shams (TDKS) equation using the pump and probe field as external fields.

Speaker: Mukhtar Muhammad Lawan (Max Planck Institute for the structure and dynamics of Matter)

13:00 Simulations of X-ray spectra of excited state processes

The theoretical characterization of the time-resolved X-ray spectra in solution requires simulation of the correlated electronic and nuclear motion with the help of advanced electronic structure methods. The goal behind this work is to use different quantum mechanical techniques to provide a realistic depiction of the coupled electronic and nuclear dynamics. For an instance, ab initio molecular dynamics along with surface-hopping dynamics will be used for simulating electronic ground state and photo-induced excited states along with extended models for adequate modelling of the solvent degrees of freedom. For a precise description of the X-ray induced core level spectra, time dependent density functional theory will be implemented.
The devised theoretical model will not only provide insight into the dynamics of charge-transfer reactions in solutions but also modelling of time-resolved X-ray spectroscopy.

Speaker: Sambit Das (Stockholm University)

13:00 Study of carrier transport in materials using time-resolved spectroscopy with ultrashort x-rays pulses

The development of technology in materials science, for instance, semiconductors technology, nowadays requires a deep understanding of the characteristics of materials with unprecedented temporal and spatial resolution. A possible experimental approach for gaining this knowledge is near-edge X-ray absorption fine structure (NEXAFS).
My Ph.D. project aims to further the research on the ultrafast photophysics of semiconductors on the few-femtosecond and even attosecond time scale by transient absorption and reflectivity spectroscopy in the XUV.
For this, the current Optical Parametric Amplifier (OPA) source available at CNR-IFN in the near IR will be optimized for the generation of isolated attosecond X-ray pulses by high-order harmonic generation in a chip obtained by femtosecond laser micromachining. Several stabilization feedback loops, such as beam pointing stabilization and long-term carrier-envelope phase stabilization will be implemented on the OPA source, which is crucial not only for generating the desired pulses, but also to increase the signal-to-noise ratio of the ongoing experiment.
A setup for transient absorption and reflectivity in the water window will be implemented and tested on different kinds of semiconductors.

Speaker: Bogdan-Constantin Ispas

13:00 Two-dimensional Hafnium and Zirconium Ditellurides: Synthesis and Electronic Band Structure Studies

The main aim of this project concerns the epitaxial growth of 2D layered transition-metal dichalcogenides (TMCs) and their structural and electronic characterization by means of in-situ scanning tunneling microscopy (STM), reflection high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES). Among other 2D TMCs, three candidate compounds, namely HfTe2, ZrTe2 and their alloy HfxZr1-xTe2, are of particular interest since they are shown to possess non-trivial topological properties [1]–[3]. Therefore, epitaxial growth of HfTe2 on three different substrates (InAs, single-layer graphene SLG and Au) using molecular beam epitaxy (MBE) has been performed, with the intention to investigate the challenges related to the growth process, as well as the effect of each substrate on the growth, structure and electronic properties of the grown materials. In that direction, questions related to the growth mode, film uniformity, substrate surface coverage, layer thickness, epitaxial quality, phase/stoichiometry and topological properties were addressed.

References:
[1] S. Aminalragia-Giamini, J. Marquez-Velasco, P. Tsipas, D. Tsoutsou, G. Renaud, and A. Dimoulas, “Molecular beam epitaxy of thin HfTe2 semimetal films,” 2D Mater., 2017.
[2] P. Tsipas et al., “Massless Dirac Fermions in ZrTe2 Semimetal Grown on InAs(111) by van der Waals Epitaxy,” ACS Nano, 2018.
[3] S. Fragkos, P. Tsipas, E. Xenogiannopoulou, Y. Panayiotatos, and A. Dimoulas, “Type-III Dirac fermions in HfxZr1−xTe2 topological semimetal candidate,” J. Appl. Phys., vol. 129, no. 7, p. 075104, 2021.

Speaker: Evgenia Symeonidou (NCSRD)

13:00 Ultrafast charge carrier dynamics in oxide semiconductors by time-resolved soft X-ray absorption spectroscopy

Hydrogen production through solar water splitting is a clean solution to the actual energy demand. It can be carried out in a photoelectrochemical cell where a photoelectrode absorbs sunlight producing electron-hole pairs, then the charge carriers are collected separately to oxidize oxygen and reduce hydrogen from water. Several materials have been proposed and tested as photoelectrodes, being n-type oxide semiconductors the most suitable as photoanodes and p-type oxide semiconductors as photocathodes. However, the development of solar water splitting is still a challenge due to its low experimental efficiency. Therefore, a fundamental understanding of the charge carrier dynamics in oxide semiconductors is mandatory to improve the current efficiency. In that sense, time-resolved soft X-ray absorption spectroscopy is a technique capable to track the dynamics of electrons and holes at the same time with element selectivity, oxidation- and spin-state specificity making it adequate to give new insights of ultrafast carrier dynamics occurring in oxide semiconductors.
The project aims to develop of a novel methodology to explore ultrafast charge carrier dynamics in oxide semiconductors by time-resolved soft X-ray absorption spectroscopy. The main goal is to locally explore the transient electronic structure of p-type (Cu$_{2}$O and CuO) and n-type (α-Fe$_{2}$O$_{3}$ and WO$_{3}$) semiconductor thin films following NIR and UV excitation by monitoring the change of X-ray absorption spectrum at specific atomic edges (M- and L-edges). Additionally, the role of dopants as Ti in modifying the dynamics in α-Fe$_{2}$O$_{3}$ will be explored.

Speaker: Miguel Omar Segovia Guzmán (Max Born Institute)

13:00 ULTRAFAST X-RAY SPECTROSCOPY OF PEROVSKITE SEMICONDUCTORS

With rapid technological developments in the last twenty years, semiconductors’ functionality in various applications has led the scientific community to intensive study of perovskites. In particular, the focus is on halide perovskites, since they display an attractive set of properties for radiation detection devices, solar cells and photovoltaic systems.
As such, my PhD project will focus on the investigation of photo-induced ultrafast dynamics in perovskite semiconductors, namely MAPbI3 and MAPbBr3, by femtosecond Near Edge X-ray Absorption Fine Structure (NEXAFS). By opting for this spectroscopic technique, I expect to gain insight in the photo-induced ultrafast dynamics triggered in these materials by femtosecond visible and UV pump. Particular emphasis will be given on the dependence of the photo-physical properties and charge transport properties on the chemical composition and the presence/type of defects in perovskite semiconductors. What is more, changes in carrier dynamics between 3D and 2D perovskites can be of great interest and are included in the project plans.
This project will be implemented and described within the SMART-X network. SMART-X is a multidisciplinary and inter-sectoral MSCA-ITN network aiming at the training of scientists who will bring X-ray ultrafast spectroscopy beyond the current state of the art as a standard technique for investigation in crucial sectors of material science, with a particular emphasis on the investigation of charge carrier dynamics in materials relevant for novel energy supply and storage.

Speaker: Ms Stavroula Vovla (Dipartimento di Fisica, Politecnico di Milano, Milano, Italy / Istituto di Fotonica e Nanotecnologie, Consiglio Nazionale delle Ricerche, Milano, Italy)

14:30 → 15:00 Afternoon Coffee

15:00 → 16:00 Supervisory Board Meeting

Friday 9 April

09:00 → 09:45 Imaging ultrafast dynamics in isolated nanoparticles

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.

Speaker: Prof. Daniela Rupp (ETH Zürich, Switzerland)

09:45 → 10:15 Approaches towards a novel sCMOS camera platform for high repetition rate X-ray imaging and single photon detection

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.

Speaker: Dr Conrad Friedrich (greateyes GmbH)

greateyes - SMART-X Stockholm Network Symposium.pdf

10:15 → 10:45 Coffee Break

10:45 → 11:15 Shaping ultrafast pulses with liquid crystals

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.

Speaker: Aurélie Jullien (Institute de Physique de Nice, UCA, CNRS)

11:15 → 12:20 ESR Round table discussion

12:20 → 12:30 OSA Chapters

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.

Speaker: Yann Amouroux (OSA Europe)

12:30 → 12:40 Concluding remarks

Speakers: Caterina Vozzi (CNR-IFN), Markus Kowalewski (Stockholm University), Michael Odelius (Stockholm University)