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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.

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; 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...

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.

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.

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

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.

A recently developed method to the full quantitative analysis of the XAS spectra extending from the absorption edge to the high-energy region is presented. This method is based on the use of two independent approaches to the analysis of the EXAFS and XANES data, the well-known GNXAS and the newly developed MXAN procedures. Herein, we report the application of this technique to two iron complexes of known structure where multiple-scattering effects are prominent, the potassium hexacyanoferrat(II) and -(III) crystals and aqueous solutions. The structural parameters obtained from refinements using the two methods are equal and compare quite well with crystallographic values. Small discrepancies between the experimental and calculated XANES spectra have been observed, and their origin has been investigated in the framework of non-muffin-tin correction. The ligand dependence of the theoretical spectra has been also examined. Analysis of the whole energy range of the XAS spectra has been found to be useful in elucidating both the type of ligands and the geometry of iron sites. These results are of particular use in studying the geometrical environment of metallic sites in proteins and complexes of chemical interest.

The first quantitative analyses are reported of the FeK-edge polarized X-ray absorption near-edge structure (XANES) of a single crystal of the iron protein carbonmonoxy-myoglobin (MbCO) and of its cryogenic photoproduct Mb*CO. The CO—Fe–heme local structure has been determined using a novel fitting procedure, namedMXAN, which is able to fit the XANES part (from the edge to about 200 eV) of experimental X-ray absorption data. This method is based on the comparison between the experimental spectrum and several theoretical spectra that are generated by changing the relevant geometrical parameters of the site around the absorbing atom. The theoretical spectra are derived in the framework of the full multiple-scattering approach. TheMXANprocedure is able to recover information about the symmetry and atomic distances, and the solution is found to be independent of the starting conditions. The extracted local structure of Mb*CO includes an Fe—CO distance of 3.08 (7) Å, with a tilting angle between the heme normal and the Fe—C vector of 37 (7)° and a bending angle between the Fe—C vector and the C—O bond of 31 (5)°

The pre-edge region of Ti N-edge polarized XANES spectra in TiO2-mtile is investigated by full-potential calculations based on the finite-difference method. Both dipolar and quadrupolar transitions are considered. The use of"non muffin-tin" potential allows a clear interpretation of the pre-edge features. The results are consistent with Full-potential LAPW band structure calculations, and are also compared with multiple-scattering calculations.

Trabajo presentado a la 15th International Conference on X-ray Absorption Fine Structure (XAFS15), celebrada del 22 al 28 de julio de 2012 en Pekín (China).-- Licencia Creative Commons Reconocimiento 3.0, We have performed a deep spectroscopic study of incommensurate satellite reflections (h, k, l) ± (0,0.278, 0) on a TbMnO3 single crystal in order to shed light on its physical origin. Resonant x-ray scattering experiments at the Mn K-edge and Tb L3-edge have been performed at different temperatures and azimuthal angles, analyzing the outgoing light polarization. Our experimental results agree with a magnetic origin for A and C-type reflections, while F and G-type have the hallmarks of anisotropic tensor susceptibility (ATS) reflections, so that they come purely from the structural modulation., The authors thank ESRF for granting beam time, especially ID20 staff. Financial support from Spanish MICINN (project FIS08-03951 and MAT2007-61621) and DGA (CAMRADS) is acknowledged.

International audience; We explore the convergence of the multiple scattering series by studying the spectral radius of the corresponding multiple scattering matrix. The energy variations of this spectral radius exhibit strong oscillations. These oscillations are shown to depend strongly on the electronic and crystallographic structure in the low energy regime. As the calculation of the eigenvalues of the multiple scattering matrix is very long, we devise a fast algorithm to approximate this spectral radius.

In this paper we present a detailed quantitative analysis of the iron K-edge XANES data of potassium ferrocyanide in water solution. The right structural determination as been obtained from the XANES data by using the MXAN procedure to fit the experimental data despite the presence of small discrepancies between experimental data and best-fit calculations at low energy. The origin of such discrepancies is also discussed in terms of non muffin-tin corrections and the role of the right charge density of the cluster.

We report the experimental extraction of the multiple-scattering contribution to the K-edge x-ray-absorption near-edge structure spectrum (XANES) of crystalline silicon. The multiple-scattering signal, is obtained by taking the difference ${\mathrm{\ensuremath{\chi}}}_{\mathrm{expt}}$(k)-${\mathrm{\ensuremath{\chi}}}_{2}$(k)=${\mathrm{\ensuremath{\chi}}}_{\mathrm{MS}}$(k)= where ${\ensuremath{\chi}}_{\mathrm{expt}(\mathrm{k})}$ is defined as [\ensuremath{\alpha}(k)-${\ensuremath{\alpha}}_{0}$(k)]/${\ensuremath{\alpha}}_{0}$(k), ${\ensuremath{\chi}}_{2}$ is the spherical-wave-calculated extended x-ray-absorption fine-structure (EXAFS) signal, and the ${\ensuremath{\chi}}_{n}$'s are the multiple-scattering contributions to the total absorption coefficient \ensuremath{\alpha}(k) assumed to have the form \ensuremath{\alpha}(k)=${\mathrm{\ensuremath{\alpha}}}_{0}$(k)[1+ over the full wave-vector range, ${\ensuremath{\alpha}}_{0}$(k) being the atomic absorption coefficient. The ${\ensuremath{\chi}}_{2}$ term has been calculated over the full energy range by using a spherical-wave formula and including mean-free-path and Debye-Waller effects. By using the subtraction procedure we find a ${\ensuremath{\chi}}_{\mathrm{MS}}$ oscillation of large amplitude in the first 70 eV of the spectrum. Comparison with the theoretical double scattering ${\ensuremath{\chi}}_{3}$(k) gives account of the main features of the ${\ensuremath{\chi}}_{\mathrm{MS}(\mathrm{k})}$ spectrum. We show that the main ${\ensuremath{\chi}}_{3}$ contribution comes from the double scattering paths involving atoms of the first as well as of the second shell and that higher terms of the series (ng3) are damped by the finite core hole lifetime and the inelastic losses of the photoelectron in the final state. Finally we analyze the K-edge absorption spectra of amorphous silicon. We find that the ${\ensuremath{\chi}}_{\mathrm{MS}}$ signal, contrary to the crystalline case, is suppressed. Its suppression is assigned to the orientational disorder in bond angles in the amorphous phase.

A theoretical calculation of the K-edge spectrum of silicon was performed. A good fit with the experimental EXAFS spectrum has been obtained by using the spherical wave formalism, the actual values of the mean free path and the Debye-Waller terms.The relevant multiple scattering contribution in the first 70 eV of the spectrum has been obtained by subtraction of a simulated EXAFS spectrum from the experimental one. INTRODUCTION. - In order to establish the role of the basic physical processes in the EXAFS and XANES pans of K-edge absorption spectra such as inelastic losses, thermal vibration and multiple scattering we have performed a careful quantitative analysis of the silicon K-edge absorption spectrum. A good fitting of the EXAFS oscillations from 50 to 450 eV above the absorption threshold (in the range of wave vector 4-11 A-l, with ET=1838 eV) has been obtained using spherical The spectrum of the mean free path h(E) of the photoelectron has been determined from photoemission experiments(3) and it has been included in the EXAFS calculations. The EXES Debye-Waller factors (4) have been also considered. They have been calculated for each shell using a dispersive model of lattice vibrations of Walker and eati in^(^) starting from X-ray diffraction measures on vibrational amplitudes in Silicon (6J). An investigation on multiple scattering effects has been carried out by subtracting the calculated spectrum from the experimental one; the difference spectrum shows an oscillation which can be explained considering the first path of multiple scattering. E