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International audience; We have derived a new Young’s formula for the polarization-averaged molecular frame photoelectron angular distributions (PA-MFPADs), as a bond length ruler for dissociating hetero-diatomic molecules. The relative errors against numerical results were within 5 %. © 2022 The Author(s).

Gold (Au) clusters with well-defined geometrical structures have attracted substantial attention due to their unique optical and catalytic properties, which are drastically changed by the ligands, ...

Imaging ultrafast hydrogen migration with few- or sub-femtosecond time resolution is a challenge for ultrafast spectroscopy due to the lightness and small scattering cross-section of the moving hydrogen atom. Here we propose time- and momentum-resolved photoelectron diffraction (TMR-PED) as a way to overcome limitations of existing methodologies and illustrate its performance in the ethanol molecule. By combining different theoretical methods, namely molecular dynamics and electron scattering methods, we show that TMR-PED, along with a judicious choice of the reference frame for multi-coincidence detection, allows for direct imaging of single and double hydrogen migration in doubly-charged ethanol with both few-fs and A resolutions, all the way from its birth to the very end. It also provides hints of proton extraction following H2 roaming. The signature of hydrogen dynamics shows up in polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) as moving features that allow for a straightforward visualization in space.

International audience; A number of renormalization schemes for improving the convergence of multiple scattering series expansions are investigated. Numerical tests on a small Cu(111) cluster demonstrate their effectiveness, for example increasing the rate of convergence by up to a factor 2 or by transforming a divergent series into a convergent one. These techniques can greatly facilitate multiple scattering calculations, especially for spectroscopies such as photoelectron diffraction, Auger electron diffraction, low energy electron diffraction , where an electron propagates with a kinetic energy of hundreds of eV in a cluster of hundreds of atoms.

We investigated the signature of ultrafast hydrogen dynamics in Polarization-Averaged Molecular-Frame Photoelectron Angular Distributions and studied intramolecular hydrogen migrations by the spectra for CH3OH2+, C2H5OH2+ and C3H7OH2+.

We derived analytical formulas of core level photoemission time delay from heteronuclear diatomic molecules and discussed their behaviors in the high energy regime, comparing them with the theoretical results for C(1s) photoemission from CO molecules.

Modern XAFS (X-ray Absorption Fine Structure) data-analysis is based on accurate multiple-scattering (MS) calculations of the X-ray absorption cross-section. In this paper, we present the inclusion and test of relativistic corrections for the multiple-scattering calculations within the GnXAS suite of programs, which is relevant to the treatment of the XAFS signals when atoms with high atomic number are contained into the system. We present a suitable strategy for introducing relativistic corrections without altering the basic structure of the programs. In particular, this is realized by modifying only the Phagen program calculating the atomic absorption cross sections and scattering t-matrices for the selected cluster. The modification incorporates a pseudo-Schrödinger Equation (SE) replacing the Dirac relativistic form. The phase-shift calculations have been put to a test in two known molecular and crystalline cases: molecular bromine Br2 and crystalline Pb. Calculations in an extended energy range have been shown to be very close to the non-relativistic case for Br2 (Br K-edge) while corrections have been found to exceed 25% for amplitude and phases of the XAFS multiple-scattering signals (Pb L3-edge). Benefits in the structural refinement using relativistic corrections are discussed for crystalline Pb at room temperature.

We present a theoretical study of the polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) emitted from the 1s orbital of oxygen atoms of dissociating dicationic carbon monoxide CO2+. Due to the polarization average, the contribution of the direct wave of the photoelectron, which represents the largest contribution to the MFPADs, is removed, so that the PA-MFPADs clearly show the details of the scattering image of the photoelectron. As a result, it is necessary to employ an accurate theory for the theoretical analysis of the continuum state. In this study, we apply a full-potential multiple scattering theory, where the space is partitioned into Voronoi polyhedra and truncated spheres, to take into account the electron charge density outside the physical atomic spheres. We do not use the spherical harmonic expansion of the cell shape functions to avoid divergence problems. The potentials in the scattering cells are computed using the multiconfigurational second-order perturbation theory restricted active space method to take into account the influence of the core hole in the electron charge density in the final state, so that a realistic relaxation can be achieved. We show that the full-potential treatment plays an important role in the PA-MFPADs at a photoelectron kinetic energy of 100 eV. In contrast, the PA-MFPADs are not sensitive to any type of major excited states in the Auger final state. We also study the dynamics of the CO2+ dissociation. We find that the PA-MFPADs dramatically change their shape as a function of the C–O bond length.

X-ray absorption spectra of vanadium pentoxide (${\mathrm{V}}_{2}{\mathrm{O}}_{5}$) at the vanadium $K$ and $L$ edges are studied with multiple scattering calculations and good agreement with experiment is obtained when the full-potential and multichannel extensions of multiple scattering theory are used. The spectral shapes are analyzed in terms of the local electronic structure at the vanadium sites and they reveal strong anisotropy and final-state correlation effects. The strong polarization dependence of the spectra is explained by the low-symmetry ligand field and vanadyl bonding. The effect of oxygen vacancies, the major intrinsic defect type of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$, is investigated. Clear spectral changes are found for vanadium atoms next to an oxygen vacancy, which may serve as a fingerprint of defects in electron energy loss spectroscopy.

International audience; Understanding the excited state of photocatalysts is significant to improve their activity for water splitting reaction. X-ray absorption fine structure (XAFS) spectroscopy in X-ray free electron lasers (XFEL) is a powerful method to address dynamic changes in electronic states and structures of photocatalysts in the excited state in ultrafast short time scales. The ultrafast atomic-scale local structural change in photoexcited WO3 was observed by W L1 edge XAFS spectroscopy using an XFEL. An anisotropic local distortion around the W atom could reproduce well the spectral features at a delay time of 100 ps after photoexcitation based on full potential multiple scattering calculations. The distortion involved the movement of W to shrink the shortest W–O bonds and elongate the longest one. The movement of the W atom could be explained by the filling of the dxy and dzx orbitals, which were originally located at the bottom of the conduction band with photoexcited electrons.

International audience; We characterize the structure of two Cu(II) complexes of 7-hydroxycoumarins in organic solvents. The solvents are, dimethyl sulfoxide and dimethylformamide. X-ray absorption spectroscopy together with density functional theory calculations are employed to identify the structural changes induced by the two solvents in comparison to the solid form of complexes. We show that the structure of the Cu(II) complexes is modified depending on the solvent and we propose the geometry of the complexes molecule.

Ultrafast excitation of photocatalytically active BiVO4 was characterized by femto- and picosecond transient X-ray absorption fine structure spectroscopy. An initial photoexcited state (≪500 fs) changed to a metastable state accompanied by a structural change with a time constant of ∼14 ps. The structural change might stabilize holes on oxygen atoms since the interaction between Bi and O increases.

Understanding the excited state of photocatalysts is significant to improve their activity for water splitting reaction. X-ray absorption fine structure (XAFS) spectroscopy in X-ray free electron lasers (XFEL) is a powerful method to address dynamic changes in electronic states and structures of photocatalysts in the excited state in ultrafast short time scales. The ultrafast atomic-scale local structural change in photoexcited WO

The local structure of Tellurium nanoparticles synthesized by arc plasma were studied by X-ray absorption spectroscopy. Nanoparticles were characterized by using different techniques to reveal particles size, morphology and composition. Te K-edge EXAFS spectra reveal a structure similar to trigonal Te, where both the inter-chain and intra-chain bonds, typical of Te crystal structure, are present. Evolution of the Debye-Waller factor with temperature reveals an increased static disorder in the nanoparticles respect to the trigonal material.

The occurrence of scissors modes in crystals that have deformed ions in their unit cells was predicted some time ago. The theoretical value of their energy is rather uncertain, however, ranging between ten and a few tens of eV, with the corresponding widths of 10−7to 10−6 eV. Their observation by resonance fluorescence experiments therefore requires a photon spectrometer covering a wide energy range with a very high resolving power. Here, a new experiment is proposed and discussed in which such difficulties are overcome by measuring with a superconducting quantum interference device (SQUID) the variation of the magnetic field associated with the excitation of scissors modes.

This contribution presents some developments of the gnxas methodology and suite of programs, providing full analysis of raw experimental XAS data through advanced multiple-scattering simulations. The main features of the gnxas suite of programs, including the basic theoretical background based on an n-body expansion of the x-ray absorption cross section, as well as useful information about the gnxas flow diagram and practical usage are reviewed. The new gnxas graphical interface w-gnxas is specifically illustrated. The recent implementation of Reverse Monte Carlo algorithms allowing for multiatomic structural refinement into the rmc-gnxas package is also presented in detail with specific examples.

We present an interface package, called es2ms (Xu J et al. Comput Phys Commun, 203:331, 2016, [1]) for passing self-consistent charge density and potential from Electronic Structure (ES) codes to Multiple Scattering (MS) codes. MS theory is based on the partitioning of the space by atomic-size scattering sites, so that the code provides the charge densities and potentials for each scattering site. For pseudopotential codes, the interface solves Poisson equation to construct the all-electron potential on the radial mesh which is used to solve the transition operators (T-matrix) and Green’s functions in MS codes. For Gaussian basis set programs, we solve recurrence relations to obtain the Coulomb potential. We describe the algorithms of the interface and an example.

We show how to implement a Full Potential Multiple Scattering (fpms) code based on a real-space FPMS theory valid for both continuum and bound states, under conditions for space partitioning that are less restrictive than those applied so far. This theory is free from the need to expand cell shape functions in spherical harmonics or to use rectangular matrices. Tests of the program show that it is able to reproduce with very good accuracy known solutions of the Schrodinger equation. Applications to the spectroscopy of low dimensional systems, such as one-dimensional (1D) chain like systems, 2D layered systems and 3D diamond structure systems, where the Muffin-Tin approximation is known to give very poor results, show a remarkable improvement toward the agreement with experiments. The default mode of the code uses superimposed atomic charge densities, which works satisfactorily in most of the applications, but with help of the es2ms interface, incorporated in the program, one can also use self-consistent charge densities derived from the vasp program. The program is also incorporated in the photoelectron diffraction code msspec and parallelized for energy point.

International audience; Ballistic Electron Emission Microscope (BEEM) is a microscope to investigate Schottky barrier based on Scanning Tunneling Microscope (STM) setup. The theoretical scheme widely used for STM is mostly focusing on an electric current from the tip tunneling through the vacuum to the sample surface. However, this model is not applicable for BEEM, since in the BEEM case, electrons tunneling through the vacuum are transported in the material over a very long range. We propose a theoretical model based on the real space full potential multiple scattering theory in order to describe this transport phenomena within the one electron picture. It is analogous to the theoretical model of angle resolved photoemission, except that the electron is emitted from the tip. This framework describes the tunneling effect and the multiple scattering in the tip and the sample and between them. Moreover this theory can be applied for non-Hermitian Hamiltonian, so that the loss of electrons at the Schottky barrier can be mimicked by introducing an imaginary part in the optical potential. © 2018, Springer International Publishing AG, part of Springer Nature.

Ultrafast excitation of photocatalytically active BiVO

International audience; Angle resolved photoemission spectroscopy (ARPES) gives detailed information about the surface electronic structures and oriented molecules. In the so-called orbital tomography method, the ARPES intensity is obtained by a simple Fourier transform of the initial state molecular orbital. While this method has given good results for a number of π-orbital systems, it is known to have strong limitations because the final state is approximated as a plane wave and thus photoelectron scattering is completely neglected. Here we propose an ARPES theory where the initial states are taken from a quantum chemistry method and the final states are calculated in multiple scattering theory. The initial molecular orbitals are reexpanded in a multi-site spherical wave basis and the multiple scattering potential is constructed from the ab initio charge density. These computations are performed algebraically using newly developed algorithms. The method is applied to pentance and the results are compared with the plane wave approximation. © 2017

International audience; The incorporation of arsenate (As(V)) in the gypsum structure is an important process for arsenic (As) fixation during industrial effluent treatment and may influence the mobility and bioavailability of As in surface environment. However, spectroscopic evidence is still lacking for its species and local structure. The species and local structure of As(V) in a gypsum lattice were investigated using Fourier transform infrared (FTIR) spectroscopy, density functional theory (DFT) modelling, and full-potential multiple scattering (FPMS) simulations. Ascorbic acid-treated As(V)-gypsum co-precipitates were used to avoid the influence of amorphous calcium arsenate on the characterization. The lack of the FTIR band in the range of 750-860 cm(-1) was an indicative that no AsO43- species was incorporated into the gypsum structure. DFT calculations proved that the incorporation of AsO43- was energetically much harder than HAsO42- species. The FPMS structural refinement yielded the optimal As-O interatomic distances of 1.77, 1.67, 1.65, and 1.66 angstrom, with an average of 1.69 +/- 0.057 angstrom, in agreement with the DFT and EXAFS results. Our work conclusively showed that HAsO42- dominated as the species of As(V) incorporated into the gypsum lattice, with the H atom in the HAsO42- group adjacent to water layer, regardless of pH.

International audience; The dynamics of the local electronic and geometric structures of WO3 following photoexcitation were studied by femtosecond time-resolved X-ray absorption fine structure (XAFS) spectroscopy using an X-ray free electron laser (XFEL). We found that the electronic state was the first to change followed by the local structure, which was affected within 200 ps of photoexcitation

International audience; GNXAS is a suite of programs based on multiple scattering calculations which performs a structural refinement of EXAFS spectra. It can be used for any system although it has been mainly developed to determine the local structure of disordered substances. We developed a user-friendly graphical user interface (GUI) to facilitate use of the codes by using wxPython. The developed GUI and the codes are multiplatform running on Windows, Macintosh and Linux systems, and are free shareware (http://gnxas.unicam.it). In this work we illustrate features and potentials of this newly developed version of GNXAS (w-GNXAS).

International audience; Hydrous ferric arsenate (HFA) is an important arsenic-bearing precipitate in the mining-impacted environment and hydrometallurgical tailings. However, there is no agreement on its local atomic structure. The local structure of HFA was reprobed by employing a full-potential multiple scattering (FPMS) analysis, density functional theory (DFT) calculations, and vibrational spectroscopy. The FPMS simulations indicated that the coordination number of the As Fe, Fe As, or both in HFA was approximately two. The DFT calculations constructed a structure of HFA with the formula of Fe(HAsO4)(x)(H2AsO4)(1-x)(OH))(y)center dot zH(2)O. The presence of protonated arsenate in HFA was also evidenced by vibrational spectroscopy. The As and Fe K-edge X-ray absorption near-edge structure spectra of HFA were accurately reproduced by FPMS simulations using the chain structure, which was also a reasonable model for extended X-Ray absorption fine structure fitting. The FPMS refinements indicated that the interatomic Fe-Fe distance was approximately 5.2 angstrom, consistent with that obtained by Mikutta et al. (Environ. Sci. Technol. 2013, 47 (7), 3122-3131) using wavelet analysis. All of the results suggested that HFA was more likely to occur as a chain with AsO4, tetrahedra and FeO6 octahedra connecting alternately in an isolated bidentate-type fashion. This finding is of significance for understanding the fate of arsenic and the formation of ferric arsenate minerals in an acidic environment.

We propose a comparative and critical assessment of multiple scattering expansions. The so-called multiple scattering series expansion is much used in the description of spectroscopies at higher energies. However, it is plagued with convergence problems when operated at lower energies. We compare this method to related methods that can be found in the literature, relying both on finite and infinite expansions. After discussing the pros and cons of these methods, we establish a simple alternative to multiple scattering series expansion which has a wider and faster range of convergence. [DOI: 10.1380/ejssnt.2012.599]

International audience; The x-ray absorption near-edge structure of graphene, graphene oxide, and diamond is studied by the recently developed real-space full potential multiple scattering (FPMS) theory with space-filling cells. It is shown how accurate potentials for FPMS can be generated from self-consistent charge densities obtained with other schemes, especially the projector augmented wave method. Compared to standard multiple scattering calculations in the muffin-tin approximation, FPMS gives much better agreement with experiment. The effects of various structural modifications on the graphene spectra are well reproduced. (1) Stacking of graphene layers increases the peak intensity in the higher energy region. (2) The spectrum of the C atom located at the edge of a graphene sheet shows a prominent pre-edge structure. (3) Adsorption of oxygen gives rise to the so-called interlayer-state peak. Moreover, O K-edge spectra of graphene oxide are calculated for three types of bonding, C-OH, C-O-C, and C-O, and the proportions of these bondings at 800∘C are deduced by fitting them to the experimental spectrum

The Elastic and Inelastic Scattering (EIS) beamline at the free-electron laser FERMI is presented. It consists of two separate end-stations: EIS-TIMEX, dedicated to ultrafast time-resolved studies of matter under extreme and metastable conditions, and EIS-TIMER, dedicated to time-resolved spectroscopy of mesoscopic dynamics in condensed matter. The scientific objectives are discussed and the instrument layout illustrated, together with the results from first exemplifying experiments.

FERMI is the first user dedicated seeded free-electron-laser (FEL) working in the extreme ultraviolet (XUV) and soft x-ray range. The EIS-TIMEX experimental end-station was availabe to external users since from the beginning of the user operation of the facility, in Dicember 2012. EIS-TIMEX has been conceived to exploit the unique properties of the FERMI source to study matter under extreme and metastable thermodynamic conditions. We hereby report on its basic parameters and applications, which includes very low jitter (i.e., high time resolution) pump-probe measurements.

Wet chemical reduction of metal ions, a common strategy for synthesizing metal nanoparticles, strongly depends on the electric potential of the metal, and its applications to late transition metal clusters have been limited to special cases. Here, we describe copper nanoclusters grown by synchrotron radiolysis in concert with wet chemistry. The local structure of copper aggregates grown by reducing Cu(II) pentanedionate using synchrotron x-ray beam was studied in situ by x-ray absorption spectroscopy. A detailed analysis of the XANES and EXAFS spectra, compared with DFT calculations and full-potential non-muffin-tin multiple scattering calculations, identified the nanocluster as Cu13 with icosahedral symmetry. The novel “charged” nanoclusters tightly bound to electron-donating amido molecules, which formed as a result of photo-induced deprotonation of ligand amines, were stabilized by irradiation. Monodispersive deposition of nanoclusters was enabled by controlling the type and density of “monomers”, in remarkable contrast to the conventional growth of metallic nanoparticles.

We argue that for some species of magnetic nanoparticles the macrospin can have a nonvanishing moment of inertia and then an orbital angular momentum. We represent such nanoparticles by two interacting rigid rotors one of which has a large spin attached to the body, namely by a Two Rotor Model with spin. By this model we can describe in a unified way the cases of nanoparticles free and stuck in an elastic or rigid matrix. We evaluate the magnetic susceptibilities for the latter case and under some realistic assumptions we get results in closed form. A crossover between thermal and purely quantum hopping occurs at a temperature much larger than that at which purely quantum tunneling becomes important. A comparison with some experimental data is outlined., Comment: 5 pages, 2 figures

State-of-the-art techniques for analysing X-ray absorption spectra are reviewed, with an eye to biological applications. Recent attempts to perform full spectral fitting of the XANES energy region and beyond for the purpose of structural analysis have met with encouraging success. The present paper analyses the theoretical motivations behind this success and indicates routes for future improvements. The theoretical background is not entirely new, although the point of view is, and some sections and appendices present material that the authors believe has never been published before. The aim of this paper is to provide a theoretical analysis that is as self-contained as possible.

High intensity pulses obtained by modern extreme ultraviolet (EUV) and x-ray photon sources allows the observation of peculiar phenomena in condensed matter. Experiments performed at the Fermi@Elettra FEL-1 free-electron-laser source at 23.7, 33.5, and 37.5 eV on Al thin films, for an intermediate-fluence range up to about $20\phantom{\rule{0.28em}{0ex}}\mathrm{J}/{\mathrm{cm}}^{2}$, show evidence for a nonmonotonic EUV transmission trend. A decreasing transmission up to about $5--10\phantom{\rule{0.28em}{0ex}}\mathrm{J}/{\mathrm{cm}}^{2}$ is followed by an increase at higher fluence, associated with saturable absorption effects. The present findings are interpreted within a simplified three-channel model, showing that an account of the interplay between ultrafast electron heating and saturation effects is required to explain the observed transmission trend.

Saturable absorption was recently observed in transmission measurements above the LII,III edge of pure Al thin films using ultra short x-ray pulses at a free-electron-laser (FEL) facility . [1] The high fluence reachable by FEL pulses, the shortness of the pulse duration, and the typical lifetime of the excited state, are all important factors enabling observation of the phenomenon. We devised a simplified theoretical model describing the saturation phenomenon using a three- channel model containing ground, excited and relaxed states. This phenomenological model explicitly includes the interaction between the solid and photon field in semi classical way, and the resulting non-linear coupled equation is solved numerically. We successfully applied this model to recent experimental results obtained using FEL radiation. [2,3]

This paper describes the scientific aims and potentials as well as the preliminary technical design of RUDE, an innovative tool for multi-disciplinary investigations in a wide field of scientific, technological and industrial applications. IRIDE will be a high intensity "particles factory", based on a combination of high duty cycle radio-frequency superconducting electron linacs and of high energy lasers. Conceived to provide unique research possibilities for particle physics, for condensed matter physics, chemistry and material science, for structural biology and industrial applications, IRIDE will open completely new research possibilities and advance our knowledge in many branches of science and technology. [RIDE is also supposed to be realized in subsequent stages of development depending on the assigned priorities. (C) 2013 Elsevier B.V. All rights reserved.

Saturable absorption was recently observed in transmission measurements above the L II , III edge of pure Al thin films using ultra short X-ray pulses at a free-electron-laser (FEL) facility. The high fluence reachable by FEL pulses, the shortness of the pulse duration, and the typical lifetime of the excited state are all important factors enabling observation of the phenomenon. We devised a simplified theoretical approach describing the saturation phenomenon using a three-channel model containing ground, excited and relaxed states. This phenomenological model explicitly includes the interaction between the solid and photon field in a semi-classical way, and the resulting non-linear coupled equation is solved numerically. We successfully applied this model to recent experimental results obtained using FEL radiation.

In accurate analyses of XPD spectra, the atomic potential at each site plays an important role. Hermitian potential like Hartree-Fock potential cannot describe the damping of the photoelectron wave, so that it is crucial to go beyond one-electron theory for the XPD analyses. Our optical potential is energy dependent, non-local and self-consistent based on Hedin's GW approximation. Here we apply this optical potential to the N 1s XPD spectra of N2 adsorbed on metals, and compare the results by use of different approximations.