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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.

While the photo-emission process is inherently inelastic, in reality a closer investigation reveals that some coherence effects depending on the single photon and electron self-interference are indeed observable. All hinges on the Feynman’s rule according to which, if the same final state can be reached by different pathways, the probability for its observation is obtained by taking the modulus squared of the sum of the probability amplitudes of the various paths, leading to the mechanism of self-interference. To this purpose we review the two-slit and the usual crystal (Bragg) diffraction experiments, highlighting that in both cases the interference patterns are due to the self-interference of the single incoming photon. Guided by these two examples, we construct the photo-emission equivalent of these experimental set-up in the case of delocalized initial states and discuss their related diffraction patterns using the language of Multiple Scattering (MS) Theory. In the general case this approach shows that coherent emission from atomic sites does not necessarily require their equivalence and new MS processes with different periodicity appear compared to the case of emission from a single localized site. In particular, the cross section for photo-emission from a valence band state factorizes, like in the diffraction counterpart, into an electronic part (photo-emission from a localized atom) and a modulating Bragg factor, where the momentum exchanged with the crystal involves the initial Brillouin Zone, the impinging photon and the final photoelectron wavevectors. Photo-emission from diatomic molecules is a particular case of this general framework. We show that, when the photoelectron is detected, the coherent emission from the two atomic centers realizes the analog of the Young’s two-slit experiment, whereas in the case of photo-absorption the Cohen-Fano interference oscillations observed in the absorption cross section are to be ascribed to new MS paths joining the two sites.

X-ray absorption spectroscopy (XAS) is a powerful method for obtaining electronic and structural information around a well-defined absorbing atom site of different types of matter, from biological systems to condensed materials in almost all possible thermodynamics conditions. The low energy part of the XAS spectrum, from the rising edge up to few hundreds eV, the so called XANES (X-ray absorption near-edge structure) region, is extremely rich of electronic and structural information, like, for example, the oxidation state, overall symmetry, interatomic distances and angles . In this paper we present in details the MXAN method and a new reengineered release of the code proposed in the literature some years ago which allows a complete fit of the XANES energy region in term of well-defined set of structural parameters. This approach is based on the comparison between experimental data and many theoretical calculations performed by varying selected structural parameters starting from a putative structure, i.e. from a well-defined initial geometrical configuration around the absorber. The X-ray photo-absorption cross sections are derived using full multiple-scattering theory, i.e. the scattering path operator is calculated exactly without any series expansion. In this way the analysis can start from the edge without any limitations in the energy range and polarization conditions. Here we present in detail the theoretical background behind the code and new capabilities implemented from theory in this last version of the program as the analysis of XANES data coming from time-dependent spectra. The code is provided with modern technology for rapid deployment of binaries based on Docker runtime so that, without source compiling, its execution reduces to just a command line striking for a full parallel (shared memory) run even using complex construct like memory filesystem in a transparent way to the end-user. Program summary Program Title: MXAN CPC Library link to program files: https://doi.org/10.17632/4k9363vcvh.1 Licensing provisions: GPLv3 Programming language: FORTRAN90 External routines/libraries: MKL or OpenBLAS library Nature of problem: In MXAN (Minuit XANES) program we implemented a fitting procedure based on a full Multiple Scattering (MS) theory [1] able to extract local structural information around the absorbing atom from experimental XANES data. The MXAN method has been successfully used for studying many known and unknown molecular systems, yielding structural geometries and metrics comparable to X-ray diffraction and/or EXAFS results [2] , [3] , [4] . The release of this new code paves the route to new possibilities available now in the latest version of the program, as the analysis of XANES data coming from time-dependent and structural disordered systems. Solution method: MXAN program is based on the comparison between experimental data and iterative theoretical calculations performed by varying selected structural parameters starting from a well-defined initial geometrical configuration around the absorber. The calculation of XANES spectra is performed within the so-called full MS approach, i.e. the inverse of the scattering path operator is computed exactly, avoiding any “a-priori” selection of the relevant MS paths [5] , [6] . The fit procedure is performed in the energy space without the use of any Fourier transform algorithm; polarized spectra can be easily analyzed because the calculation is performed using the full MS approach [1] . The optimization in the space of the parameters is achieved using the CERN-library MINUIT [7] routines minimizing the square residual. Additional comments including restrictions and unusual features: Depending on the molecular system under study and on the operating runtime conditions the program may or may not fit into available host RAM memory. The present release of the code is limited to a single SMP machine by running in parallel using OpenMP and/or the multi-threaded version of MKL or OpenBLAS.

We investigate the ground state and low-energy spin-orbital excitations of a single iron(II) phthalocyanine molecule in isolation and on an oxidized Cu(110) surface. Considering the subspace spanned by the three lowest spin-triplet states of 3A2g and 3Eg symmetry, we diagonalize the Hamiltonian made of the anisotropic spin-orbit interaction and the ligand field splitting Δ, defined as the energy difference between 3Eg and 3A2g. We find that the ground state switches from a 3Eg-like state with large orbital moment and out-of-plane easy axis for Δ−60 meV. The analysis of the first excited states in the two regimes explains the zero-field splitting data reported for β-FePc as well as for FePc molecules adsorbed on an oxidized Cu(110) surface [N. Tsukahara et al., Phys. Rev. Lett. 102, 167203 (2009)]. Importantly, the calculated magnetic susceptibility obtained with the ab initio value Δ=93 meV compares remarkably well with the experimental data of β-FePc in the whole available temperature range of 1–300 K., This work has been partially supported by the Spanish MAT2017-83468-R project, Aragonese E-12 17R RASMIA (co-funded by Fondo Social Europeo), and of the European Union FEDER (ES). We also acknowledge funding by the European Union COST MP1306 EUSPEC project and by the Japan Society for the Promotion of Science (JSPS), Grant KAKENHI JP 16K05393.

We determine the magnetic ground state of the FePc molecule on Au-supported thin films based on the observed values of orbital anisotropy and spectroscopic x-ray magnetic circular dichroism (XMCD) measurements at the Fe $K$ and $L$ edges. Starting from ab initio molecular orbital multiplet calculations for the isolated molecule, we diagonalize the spin-orbit interaction in the subspace spanned by the three lowest spin triplet states of ${}^{3}{A}_{2g}$ and ${}^{3}{E}_{g}$ symmetry in the presence of a saturating magnetic field at a polar angle $\ensuremath{\theta}$ with respect to the normal to the plane of the film, plus an external perturbation representing the effect of the molecules in the stack on the FePc molecule under consideration. We find that the orbital moment of the ground state strongly depends on the magnetic field direction in agreement with the sum rule analysis of the ${L}_{23}$-edge XMCD data. We calculate integrals over the XMCD spectra at the Fe $K$ and ${L}_{23}$ edges as used in the sum rules and explicitly show that they agree with the expectation values of the orbital moment and effective spin moment of the ground state. On the basis of this analysis, we can rule out alternative candidates proposed in the literature.

International audience; A brief derivation of the cross section in the independent particle approximation for some of the spectroscopies treated in the msspec program package is presented. We solve the related Schrödinger equation with a complex energy-dependent effective potential in the framework of multiple scattering theory to write the cross-section for photoemission and photoabsorption in a physically transparent way that provides insight in their interpretation and analysis. Relativistic corrections are also implemented. In order to be able to apply this theory to a wide variety of systems we use a kind of all-purpose optical potential, depending only on the local density of the system under investigation, and discuss its merits and drawbacks. A Green’s function approach is shown to be necessary to write the photoabsorption cross section in the case of complex potential. © 2018, Springer International Publishing AG, part of Springer Nature.

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.

Inorganic compounds containing light transition metal ions in a formal (\(3d^0\)) electronic configuration often have a complex atomic structure and several polymorphs may co-exist, especially in nanomaterials. X-ray absorption spectra at the metal L-edge probes the symmetry, electronic state and bonding of the transition metal ion. In low symmetry crystals, linear dichroism is common and reflects a complicated ligand field. We show that the metal \(L_{2,3}\)-edge spectra, which are dominated by strong particle-hole multiplet coupling, are well described with multichannel multiple scattering theory. The ligand field and band effects are included ab initio by the anisotropic scattering of the excited electron, whereby complex dichroism can be accounted for. Titanite nanostructures and calcium carbonate are taken as examples.

International audience; We derive a general spherical wave multiple scattering expression of the EELS cross-section. Contrarily to most of the previous theoretical frameworks, this approach treats all the electrons involved on an equal footing with respect to multiple scattering. The main point in our results is that the cross-section depends not only on Coulomb integrals on the absorbing atom, but also on contributions from all the other atoms. We show that these external contributions should be restricted to neighbouring atoms. © 2018, Springer International Publishing AG, part of Springer Nature.

International audience; We present a scattering theoretic approach to the calculation of the cross-section of (e,2e) impact spectroscopy where all the electrons involved are treated within the real space multiple scattering framework. This approach is particularly suited to the reflection geometry at low kinetic energies, with the ejection of a core-level electron. In this case, we expect (e,2e) spectroscopy can be turned into an extremely sensitive surface structure probe. © 2018, Springer International Publishing AG, part of Springer Nature.

International audience; We present an interface package, called ES2MS, 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 pseudo potential 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. We show the algorithm of the interface and the example for X-ray absorption spectra of graphene.

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

Under the terms of the Creative Commons Attribution license.-- et al., The observation of an anomalous quadrupolar signal in x-ray magnetic circular dichroism (XMCD) at the Fe K-edge of iron phthalocyanine (FePc) films is reported. All ground states previously suggested for FePc are incompatible with the experimental data. Based on ab initio molecular orbital multiplet calculations of the isolated FePc molecule, we propose a model for the magnetic ground state of the FePc film that explains the XMCD data and reproduces the observed values of the orbital moments in the perpendicular and planar directions., The financial support of the Spanish financial agency MINECO MAT2011-23791 and MAT2014-53921-R, Aragonese DGA-IMANA E34 (co-funded by Fondo Social Europeo), and European Union FEDER funds is acknowledged. The research at UCSD was supported by the Office of Basic Energy Science, US Department of Energy, BES-DMS, funded by the Department of Energy Office of Basic Energy Science, DMR, under Grant No. DE FG03 87ER-45332.

Scanning tunneling microscopy and x-ray absorption spectroscopic results at the Fe K edge of Fe phthalocyanine (FePc) thin films grown on Au substrates, together with theoretical calculations, allow us to refine the structure of the film. In particular, we show that the columnar stacking of the FePc molecules is different from that found in bulk ¿ and ß phases. Moreover, the molecules do not lay parallel to the surface of the substrate. These structural findings are relevant to understand magnetism of FePc films., The financial support of the Spanish financial agency MINECO MAT2011-2379 and MAT2014-53921-R, Aragonese DGA-IMANA E34 (co-funded by European Social Fund), as well as European Union FEDER funds is acknowledged. The sample preparation and initial structural characterization were supported by the Office of Basic Energy Science, U.S. Department of Energy, BES-DMS funded by the Department of Energy’s Office of Basic Energy Science, DMR under Grant No. DE FG03 87ER-45332 and NSF DMR 0847552.

A new method for calculating X-ray absorption spectroscopy (XAS) at the L2,3 edges of Ca and transition metals is presented. It is based on the multichannel multiple-scattering theory by Natoli et al. [Phys. Rev. B, (1990), 42, 1944-1968] combined with the eigen-channel R-matrix formalism. Atomic multiplet-like effects, owing to the Coulomb interaction of photoelectrons and the 2p hole, are taken into account through a configuration interaction ansatz for the final-state wavefunction. The various multiplet states lead to a set of channels for the photoelectron wavefunction, which is calculated in multiple-scattering theory. The method is applied to Ca, an important element for biological applications of XAS. An L3:L2 branching ratio of 3:4 is found, in good agreement with experiment but in contrast to the statistical value 2:1 obtained in all one-electron approaches. By using a linear mixture between statically screened (approximately 90%) and unscreened (approximately 10%) core-hole potential, the line shape, too, agrees well with the experimental one.

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.

X-ray natural circular dichroism (XNCD) has been recently detected in the XANES region for uniaxial and biaxial gyrotropic crystals. Chiral-EXAFS (χ-EXAFS) spectra are reported for the first time over a wider energy range and are analysed in terms of multiple-scattering paths of relevant symmetry. For such heavily absorbing single crystals as lithium iodate, paratellurite or potassium titanyl phosphate, the differential absorption between left-handed and right-handed circularly polarized X-ray photons cannot be measured in transmission but gyrotropy effects can still be detected in fluorescence excitation spectra. Whereas XNCD and fluorescence-detected X-ray natural circular dichroism spectra are strictly identical for uniaxial crystals, it has been established that this was true only to the first order for biaxial crystals such as potassium titanyl phosphate.

Difference EXAFS and XANES studies at the Mn K-edge are reported which shed light on the mechanisms of activation of the title compounds by ozone. The reaction of ozone with either the chloro- or b...

The experimental evidence for orbital ordering in ${\mathrm{LaMnO}}_{3}$ and ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}{\mathrm{MnO}}_{4}$ is critically reexamined in the light of realistic calculations based on multiple scattering theory and finite difference method for the resonant x-ray scattered intensity at a forbidden reflection. It is found that the dipole resonant x-ray diffraction at the Mn $K$-edge is not a suitable technique for the direct observation of the orbital ordering in the considered compounds.

We present the first successful attempt at calculating cluster full-potential x-ray absorption near-edge structure (XANES) spectra, based on the finite difference method. By fitting XANES simulations onto experimental spectra we are able to perform electron population analysis. The method is tested in the case of Ti $K$-edge absorption spectrum in ${\mathrm{TiO}}_{2}$, where the amount of charge transfer between Ti and O atoms and of the screening charge on the photoabsorber is obtained taking into account both dipolar and quadrupolar transitions.

A method is presented to calculate the natural circular dichroism recently discovered in the X-ray range (XNCD). The basic formula represents XNCD as an odd second-rank tensor and leads to a sum rule that relates XNCD to the mixing of odd and even orbitals in the ground state. A multiple-scattering theory of XNCD is presented, and calculated spectra for the L-edges of iodine in LiIO3 compare favorably with the experiments.

In analyzing the x-ray-absorption near-edge structure (XANES) at the La, Nd, Gd, Y, and Sr ${L}_{3}$ edges in $T$- and ${T}^{\ensuremath{'}}$-phase materials, we have found a general rule that links the presence of a feature at about 20 eV from the rising edge, called $A$ in the experimental data by Tan et al. [Phys. Rev. Lett. 64, 2715 (1990)], with the geometrical arrangement around the absorbing site. The absorption spectra of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4},$ ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4},$ ${\mathrm{Gd}}_{2}{\mathrm{CuO}}_{4},$ and ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ compounds are analyzed by full multiple-scattering theory with complex potential of Hedin-Lundqvist type obtaining very good agreement between theory and experiments. As a result of this analysis we show that feature $A$ in these spectra appears every time the photoabsorber has an eightfold hexahedron-type coordination with its nearest neighbors. This finding can be used to determine the rare-earth ordering in ${T}^{*}$-phase superconductors.

We produce the first experimental evidence of x-ray natural circular dichroism (XNCD) in a uniaxial gyrotropic crystal of α-LiIO3 that is known to crystallize with space group P63 and to exhibit a very strong nonlinear susceptibility. For the sake of simplicity, the optical axis was set colinear to the direction of the exciting x-ray beam in order to get rid of undesirable birefringence and linear dichroism effects. The nicely structured XNCD spectra recorded at the LI, LII, and LIII edges of iodine are assigned to the electric dipole–electric quadrupole (E1.E2) interference terms which do not vanish in gyrotropic crystals but contribute to a second order polarizability. Our interpretation is consistent with a band structure calculation and is fully supported by ab initio multiple scattering simulations. The signatures recorded at the LII and LIII edges are similar and have the same sign. This is in contrast to x-ray magnetic circular dichroism (XMCD) spectra which usually exhibit opposite signs at the LI...

The electronic structure of the layered titanium and tantalum disulfides ${\mathrm{TiS}}_{2}$ and ${\mathrm{TaS}}_{2}$ is investigated by the experimental sulfur and titanium K-edge x-ray-absorption near-edge structures, full multiple-scattering theory, and tight-binding linear muffin-tin orbital band-structure method. Good agreement between experimental data and theoretical calculations is obtained, especially in preedge regions that are interpreted in terms of mixing between the central atom p and higher-neighboring metal d orbitals. The variation of width and intensity of the prepeak is associated with the occupation of the d band and linked with metal site symmetry.

\ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$,${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$, and FeO compounds are characterized by means of x-ray-absorption near-edge-structure spectroscopy at the oxygen K edge. Using increasing cluster sizes around the excited atom in the full multiple-scattering simulations, we are able to link the features present in the spectra of each iron oxide to its specific atomic arrangement and electronic structure. The prepeak structure is successfully reproduced and interpreted as transitions from the oxygen 1s core state to antibonding oxygen 2p states hybridized with Fe 3d orbitals. Their intensity and shape depends on the Fe site symmetry, the occupation number of the d levels, and the O-Fe bond length of each different iron oxide. Higher lying spectral features are shown to be related to scattering of the photoelectron by a particular oxygen shell and an extended x-ray absorption fine structure--like relation is established between their energy position and the distance of the corresponding shell from the photoabsorber.

We present a theoretical analysis, based on the zeroth-order approximation of the multichannel multiple-scattering theory, of the x-ray-absorption spectra of ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$, ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, and ${\mathrm{Ca}}_{2}$${\mathrm{CuO}}_{3}$ compounds at the copper K edge. Single-channel theoretical calculations have been made using atomic clusters large enough to be sure that all one-electron features have been correctly described. On this basis the presence of both 3${\mathit{d}}^{9}$ and 3${\mathit{d}}^{10}$L configurations in the ground state has been found essential to obtain good agreement between experimental data and theoretical calculations in the case of ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$ and ${\mathrm{Ca}}_{2}$${\mathrm{CuO}}_{3}$. Conversely, the best agreemnt with the ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ data has been obtained by taking into account only the 3${\mathit{d}}^{10}$L configuration in the final state. We briefly comment on this finding. \textcopyright{} 1996 The American Physical Society.

Sulfur $K$- and titanium ${L}_{2,3}$-edge absorption spectra of transition-metal disulfide, Ti${\mathrm{S}}_{2}$, were measured using x-ray-absorption spectroscopy (XAS) and electron-energy-loss spectroscopy (EELS), respectively. All features in the experimental spectra are very well reproduced by full multiple-scattering-theory calculations. Combined analysis of theoretical and experimental XAS and EELS spectra, and linear muffin-tin orbital band-structure calculations, tend to demonstrate that Ti${\mathrm{S}}_{2}$ is a semimetal.

We have studied the arsenic-terminated Si(111)-(1 × 1) surface with low energy photoelectron diffraction. On this ideally terminated surface, the Si 2p core-level consists of a bulk component and a single well separated surface component shifted to 0.75 eV higher binding energy compared to the bulk component. We have measured the intensities of the surface and bulk components of the Si 2p core-level, and the single component As 3d core-level, as functions of azimuthal angle. The recorded azimuthal scans for the Si 2p surface component showed extremely large intensity variations. For a photon energy of 130 eV and emission angle of 45° for example, the intensity of this component was found to vary by 400 percent upon an azimuthal rotation of the sample by 45°. The experimental data were compared to model calculations of the photoelectron diffraction utilizing a fully convergent multiple scattering formalism. The best fit to the data was consistently found in all the azimuthal scans for an outward relaxation of the As atoms from the Si bulk positions by 0.2 A. This is in good agreement with earlier structural determinations of this surface.

X-ray absorption calculations of Ca L23-edge spectra of calcium carbonate in its two main crystal phases, calcite and aragonite, are reported. The multichannel multiple scattering theory with a correlated particle-hole wave function and a partially screened core-hole potential is used. Very good agreement with experiment for both CaCO3 phases is obtained, while the independent particle approximation completely fails. For aragonite, appreciable linear dichroism is predicted in agreement with recent observations.

We briefly discuss how to reduce the many-body problem of the photoemission (photoabsorption) process to an effective one particle problem with complex (optical) potential. By solving the associated Schrodinger equation in the framework of the multiple scattering (MS) theory, we present a unifying approach for interpreting and calculating the X-ray absorption structure (XAS), both magnetic (MCD) and nonmagnetic, and the photoelectron diffraction patterns (PhD), which are shown to be interpretable as a kind of EXAFS in three dimensions (energy plus two angles defining the photoelectron escape direction). We shall also comment on the theoretical aspects of the photoemission and photoabsorption processes that are common to both techniques and enter into a practical code of calculation of the related signals, like the muffin-tin approximation and the way to eliminate it, the effective optical potential, the photoelectron damping, the role of the intrinsic and extrinsic excitations.

International audience; Methodological advances in multiple scattering theory (MST) in both wave and Green's function versions are reported for the calculation of electronic ground and excited state properties of condensed matter systems with an emphasis on core-level photoemission and absorption spectra. Full-potential MST is reviewed and extended to non-local potentials. Multichannel MST is reformulated in terms of the multichannel density matrix whereby strong electron correlation of atomic multiplet type can be accounted for in both ground and excited states.

Rotational strengths of the core electron excitations of the chiral molecule propylene oxide are calculated using the random phase approximation (RPA). As expected, the predicted values are small but still in the realm of experimental detectability, having dissymmetry factors g of the order of 10 −3 . The quality of RPA calculations for core excitations using the 6-31Gpol basis set is assessed via the comparison of calculated excitation energies and oscillator strengths with experimental and theoretical results for reference molecules.

We present ab initio calculations of x-ray-absorption fine structure for the Re L 3 and L 1 edges in crystalline ReO 3 based on an exact curved-wave multiple-scattering approach. Good agreement between theoretical and experimental data has been found for both edges. We show that, as expected, the contribution of multiple-scattering signals from linear chains, like Re-O-Re and O-Re-O, is very large both in the L 3 - and L 1 -edge spectra due to the strong focusing effect caused by the middle atom

We present a general multiple-scattering (MS) scheme utilizing complex potentials for the calculation and interpretation of inner-shell x-ray-absorption fine structure for clusters of atoms in condensed matter with application to the case of the molecules SF 6 , GeCl 4 , and Br 2 . The method is based on the solution of the Dyson equation with a complex self-energy of the Hedin-Lundqvist type for the single-particle Green's function describing the propagation of the excited photoelectron through the system

International audience; Following our recent derivation of a Real Space Full-Potential Multiple-Scattering-Theory (RSFP-MST), we calculate the low-energy part of the K-edge absorption spectra of the transition metals of the first row of the periodic table using the Hedin-Lundqvist (HL) and the Dirac-Hara (DH) complex potentials, which are often used in the literature for this kind of calculations. In both cases the complex part is taken from the HL potential. Taking advantage of the fact that in the FP-MS scheme the Schrödinger Equation is solved exactly without approximations (within the limits of the l-truncation procedure in MST), we intend to test the ab initio validity of these optical potentials in cases that are borderline between the independent particle and the weakly correlated regime. Surprisingly enough, we find better agreement with experiments with the DH potential.

24 páginas, 6 figuras, 1 tabla.-- El pdf del artículo es la versión pre-print: arXiv:0809.0069v5, We present a rigorous derivation of a real-space full-potential multiple scattering theory (FP-MST) that is free from the drawbacks that up to now have impaired its development (in particular the need to expand cell shape functions in spherical harmonics and rectangular matrices), valid both for continuum and bound states, under conditions for space partitioning that are not excessively restrictive and easily implemented. In this connection we give a new scheme to generate local basis functions for the truncated potential cells that is simple, fast, efficient, valid for any shape of the cell and reduces to the minimum the number of spherical harmonics in the expansion of the scattering wavefunction. The method also avoids the need for saturating 'internal sums' due to the re-expansion of the spherical Hankel functions around another point in space (usually another cell center). Thus this approach provides a straightforward extension of MST in the muffin-tin (MT) approximation, with only one truncation parameter given by the classical relation lmax = kRb, where k is the electron wavevector (either in the excited or ground state of the system under consideration) and Rb is the radius of the bounding sphere of the scattering cell. Moreover, the scattering path operator of the theory can be found in terms of an absolutely convergent procedure in the lmax →∞ limit. Consequently, this feature provides a firm ground for the use of FP-MST as a viable method for electronic structure calculations and makes possible the computation of x-ray spectroscopies, notably photo-electron diffraction, absorption and anomalous scattering among others, with the ease and versatility of the corresponding MT theory. Some numerical applications of the theory are presented, both for continuum and bound states., CRN acknowledges financial support from DGA (Diputaci´on General de Arag´on) in the framework of the promotion action for researcher mobility. This work has been accomplished in the framework and with the support of the European Network LightNet.

Physical review / B 42(1), 37 - 42 (1990). doi:10.1103/PhysRevB.42.37, We demonstrate that texture effects can seriously distort the measurement of x-ray-absorption data from powdered specimens. We show that the isotropic average dipolar x-ray-absorption spectrum can always be measured irrespective of the degree of texture, provided axial symmetry in the particle distribution function exists normal to the specimen surface and a linearly polarized x-ray beam is employed. A simple modification to synchrotron-radiation experiments to achieve this consists of rotating the specimen by θm about an axis perpendicular to the polarization vector and beam direction from normal incidence. θm is given by sin2Θm=1/3., Published by Inst., Woodbury, NY

We present a Full Potential Multiple Scattering (FP-MS) scheme for the interpretation of several X-ray spectroscopies that is a straightforward generalization of the more conventional Muffin-Tin (MT) version. Like this latter, it preserves the intuitive description of the physical process under consideration and overcomes some of the limitations of the existing FP-MS codes. It hinges on a fast and efficient method for solving the single cell scattering problem that avoids the convergence drawbacks of the angular momentum (AM) expansion of the cell shape function and relies on an alternative derivation of the multiple scattering equations (MSE) that allows us to work reliably with only one truncation parameter, {\it i.e.} the number of local basis functions in the expansion of the global scattering function determined by the classical relation $l_{\rm max} \sim k R$., Comment: 4 pages 2 figures

We present a new scheme for solving the scattering problem for an arbitrarily shaped potential cell that avoids the well known convergence problems in the angular momentum expansion of the cell shape function. Tests of the method against analytically soluble separable model potentials, with and without shape truncation, have been performed with success. By a judicious choice of the shape of the cells partitioning the whole molecular space and use of empty cells when necessary, we set up a multiple scattering scheme that leads to a straightforward generalization of the same equations in the muffin‐tin approximation. For example lmax in the angular momentum expansion can still be chosen according to the rule lmax ∼ kR, where R is the radius of the bounding sphere of the cell and all the matrices appearing in the theory are square matrices.

Resonant X-ray scattering (RXS) is a spectroscopy where both the power of site selective diffraction and the power of local absorption spectroscopy regarding atomic species are combined. By virtue of the dependence on the core level state energy and the three dimensional electronic structure of the intermediate state, this technique is specially suited to study charge, orbital or spin orderings and associated crystal distortions. In the case of charge ordering, we exploit the fact that atoms with closely related site symmetries but with small charge differences exhibit resonances at slightly different energies. The sensitivity of this effect allows for quantitative estimations of the charge disproportion. Opposite to fluorescence or absorption measurements, the power of diffraction relies on the capability of detecting differences that are smaller than the inverse lifetime of the core hole level. To account for the uncertainty of the crystallographic structure and the fact that the charge ordering must be disentangled from the associated atomic displacements, a complete methodology is proposed and applied to the low temperature phase of magnetite. Relative sensitivity on spin, toroidal and orbital ordering is also shown and compared in different transition metal oxide compounds, like V2O3 and GaFeO3.

We report a theoretical method for x-ray absorption spectroscopy (XAS) in condensed matter which is based on the multichannel multiple scattering theory of Natoli et al. and the eigen-channel $R$-matrix method. While the highly flexible real-space multiple scattering (RSMS) method guarantees a precise description of the single-electron part of the problem, multiplet-like electron correlation effects between the photoelectron and localized electrons can be taken account for in a configuration interaction scheme. For the case where correlation effects are limited to the absorber atom, a technique for the solution of the equations is devised, which requires only little more computation time than the normal RSMS method for XAS. The method is described and an application to XAS at the Ca ${L}_{2,3}$ edge in bulk Ca, CaO, and $\mathrm{Ca}{\mathrm{F}}_{2}$ is presented.

We have investigated the pre-edge features in x-ray absorption spectra of transition metal (TM) monoxides at the metal $K$ edge. By comparing the calculated dipolar and quadrupolar spectra with experiments we assign the first peak at the lowest energy to a direct quadrupolar transition (due to the more effective attraction of the core hole). The following peaks in this region are mainly dipolar in character and reflect the density of states due to the medium range order as long as the radius of the cluster equals the cation-cation shortest distance plus the nearest neighbor cation-anion bond, representing a band-like effect arising from the hybridization of the $4p$ orbitals of central TM atoms with the $3d$-based octahedral molecular orbitals of the higher-neighboring cations.