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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.

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...

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.

\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 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.

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 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.

The FeNO bond angle in a series of {FeNO}7 complexes has been probed by EXAFS, utilizing a new theoretical data analysis package, GNXAS. This package provides an integrated approach to the analysis of EXAFS data based on a full curved-wave, multiple-scattering theoretical treatment incorporating least squares refinement. EXAFS data were obtained on two crystallographically-characterized {FeNO}7 inorganic complexes with varying FeNO angles to examine the sensitivity of the GNXAS fit to this angle. Results are presented which indicate that it is possible to determine whether the FeNO unit is bent or linear, with the GNXAS analysis being extremely sensitive when the angle is between 150° and 180°. This study thus provides the basis for the determination of the coordination geometry of molecules like No and O2 to metalloprotein active sites.

GNXAS, a recently developed integrated approach to the analysis of EXAFS data is presented in detail. GNXAS provides for the direct fitting of theoretical signals (calculated by utilizing the Hedin-Lundqvist complex exchange and correlation potential and spherical wave propagators) to the experimental data. GNXAS is able to calculate all the signals related to two-, three-, and four-atom correlation functions with the proper treatment of correlated distances and Debye-Waller factors. The technique is particularly well-suited for the analysis of multiplescattering effects and thus allows for accurate determination of bond distance and angular information of second and third neighbors. Herein we report the application of GNXAS to several chemical systems of known structure. The reliability of GNXAS was evaluated on a well-ordered inorganic complex, Fe(acac)3, as well as a lower-symmetry coordination complex with mixed ligation, Na[Fe(OHz)EDTA]. The total EXAFS signal generated by GNXAS matches closely the experimental data for both complexes, especially when all the multiple-scattering contributions were included in the theoretical signal. First neighbor distances obtained from refinement using GNXAS, as well as distances and angles for further neighbors, compared very well with crystallographic values. The angle dependence of the Fe-C-N multiple-scattering contribution in K3Fe(CN)6 was also examined. The results indicate that GNXAS can be used to determine angles relatively accurately for Fe-C-N configurations with angles greater than about 150”. These results establish the utility and reliability of the GNXAS approach and provide a reliable means to determine additional structural information from EXAFS analysis of structures of chemical interest.

The Fe-N-O bond angle in a series of [FeNO][sup 7] complexes has been probed by EXAFS, utilizing a new theoretical data analysis package, GNXAS. This package provides an integrated approach to the analysis of EXAFS data based on a full curved-wave, multiple-scattering theoretical treatment incorporating least-squares refinement. Since GNXAS is able to calculate all the signals relating to two-, three-, and four-atom correlation functions with the proper treatment of correlated distances and Debye-Waller factors, it is particularly well-suited for analysis of multiple-scattering effects and bond angle determination. EXAFS data were obtained on a series of crystallographically-characterized [FeNO][sup 7] inorganic complexes with varying Fe-N-O angles to examine the sensitivity of the GNXAS fit to this angle. The compounds studied were Fe(TMC)NO (where TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) which has an Fe-N-O bond angle of 177.5(5)[degree], Fe(TACN)(N[sub 3])[sub 2]NO (where TACN = N,N[prime],N[double prime]-trimethyl-1,4,7-triazacyclononane) which has an angle of 156(1)[degree], and Fe(salen)NO (where salen = N,N[prime]-ethylenebis(salicylideneiminato)) which has a bond angle of 127(6)[degrees] at 175[degrees]C and 147(5)[degrees] at 23[degree]C. 28 refs., 15 figs., 3 tabs.

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.

By making a comparison between the Mn K-edge absorption of (${\mathrm{MnO}}_{4}$${)}^{\mathrm{\ensuremath{-}}}$ and [Mn(${\mathrm{H}}_{2}$O${)}_{6}$${]}^{2+}$ complexes in aqueous solution we obtain an experimental determination of the energy extent of the type-II multiple-scattering (MS) regime that is substantially wider than expected. Theoretical calculations based on the MS formalism support this conclusion. We also recognize three energy regions in the absorption spectra of these complexes: a full MS region, where numerous or an infinite number of MS paths of high order contribute (depending on whether the MS series converges or not), an intermediate MS region, where only a few MS paths of low order are relevant, and a single-scattering region where the photoelectron is backscattered only once by the ligands [extended x-ray-absorption fine-structure (EXAFS) regime]. Theoretical considerations show that this must be a general situation in x-ray-absorption spectra and opens the way to a unified scheme for their interpretation. The energy extent of the three regions is obviously system dependent. We also show how to generalize to MS contributions the usual EXAFS analysis using curved-wave propagators and indicate how to extract geometrical information from the spectra of the two clusters investigated. In particular the method is used to derive the Mn---O---O---Mn path length in the (${\mathrm{MnO}}_{4}$${)}^{\mathrm{\ensuremath{-}}}$ complex.

The difference between the K-edges XANES (X-ray Absorption Near Edge Structure) spectra of crystal and amorphous silicon is discussed. We show that the multiple scattering signal gives a large contribution to the total absorption in crystalline silicon in an energy range of about 70eV. The double scattering term probing the triplet distribution function gives the major contribution to the XANES. We show that the difference between the crystalline and amorphous absorption spectra is due mainly to the crystalline multiple scattering signal which is quenched in the amorphous spectrum. The suppression of the signal due to the triplet distribution function in the amorphous silicon is assigned to the large orientational disorder.

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

The binding of Mn2+ ions to ATP molecules has been studied by means of the fine-structure analysis of X-ray absorption spectra (EXAFS and XANES) at the K-edge of Mn using the synchrotron, radiation facility PULS in Frascati. The results obtained in both freeze-dried and liquid samples of aqueous solutions at room temperature of the Mn-ATP complex under diffrent values of pH and Mn:ATP ratio are reported and discussed. The Mn2+ ion appears to be octahedrally coordinated, the phosphate oxygen atoms being in the first co-ordination shell. Values of (2.15±0.05) A and (3.4±0.05) A, respectively, have been found for Mn-O and Mn-P distances. Theoretical considerations together with experimental results do not support the existence of a direct binding of the Mn to the N(7) atom of the adenine ring. The coordination numbers for the first and second shell are consistent with the formation of a Mn(ATP)2 complex in solution at pH=9 and Mn: ATP=1:10, while a Mn-ATP 1:1 complex is found at lower pH and higher Mn: ATP ratios.