Full Multiple-scattering Approach Research Articles

Analysis of the X-ray absorption spectra near the cobalt K-edge of the Co(2,2'-Bipy)(i-Bu2PS2)2 complex

The chelate Co(2,2'-Bipy)(i-Bu2PS2)2 has been studied by X-ray absorption near-edge structure (XANES) spectroscopy. The experimental Co K-edge XANES of the Co(2,2'-Bipy)(i-Bu2PS2)2 has been recorded. Theoretical analysis of Co K-XANES has been performed on the basis of the full-potential finite difference method using FDMNES code and real-space self-consistent full multiple-scattering approach within the muffin-tin approximation for potential shape (FEFF8.4 program). The comparison of the theoretical spectra with the experimental data has been done and a agreement between the theoretical and experimental XANES has been obtained.

Geometric Structure Determination of N694C Lipoxygenase: A Comparative Near-Edge X-Ray Absorption Spectroscopy and Extended X-Ray Absorption Fine Structure Study

The mononuclear nonheme iron active site of N694C soybean lipoxygenase (sLO1) has been investigated in the resting ferrous form using a combination of Fe-K-pre-edge, near-edge (using the minuit X-ray absorption near-edge full multiple-scattering approach), and extended X-ray absorption fine structure (EXAFS) methods. The results indicate that the active site is six-coordinate (6C) with a large perturbation in the first-shell bond distances in comparison to the more ordered octahedral site in wild-type sLO1. Upon mutation of the asparagine to cysteine, the short Fe-O interaction with asparagine is replaced by a weak Fe-(H(2)O), which leads to a distorted 6C site with an effective 5C ligand field. In addition, it is shown that near-edge multiple scattering analysis can give important three-dimensional structural information, which usually cannot be accessed using EXAFS analysis. It is further shown that, relative to EXAFS, near-edge analysis is more sensitive to partial coordination numbers and can be potentially used as a tool for structure determination in a mixture of chemical species.

Theoretical calculations of K-edge absorption spectra in warm dense Al

Theoretical calculations of the shape of an absorption edge in a warm dense simplematerial such Al are described. The calculations include the XAFS (x-ray absorption finestructure) features above the edge. The shape of these modulations relates to the atomicspatial distribution and to the electronic density of states (DOS) in the continuum. Theunderlying warm dense plasma model is based on a density-functional neutralpseudoatom approach which gives the radial distribution function (RDF) and theelectron density around one specific site. Combined with a full multiple-scattering(MS) approach giving the final free wavefunction in a photoionization process, themodel is used to predict the shape of photoabsorption K edges over a range ofcompression ratios from 0.5 to 3 and temperatures from 0.1 to 10 eV. It is found thatXAFS structures are intense enough to be observable for densities of the order of(and above) the solid density and that dynamic or static absorption spectroscopymeasurements could provide interesting information on the warm dense matterregime.

X-ray absorption near-edge structure spectroscopy research on electronic structures and magnetic properties of (Fe1-xNix)2P

The Fe2P, Ni2P and (Fe1-xNix)2P compounds (x=01,025,05) are studied by X-ray absorption near- edge structure (XANES) spectroscopy at the Fe, Ni and P K-edges. Combined with full multiple-scattering approach calculations of XANES, different contributions of 3f, 3g crystallographic positions to XANES spectra for metal atoms are discussed. It was found that the Ni atom prefers to substitute Fe (3f) sites when introducing Ni atoms. The first-principle density of states calculations were performed and the results show that unoccupied P p-projected DOS of paramagnetic Fe2P and Ni2P are matched with their K-edges XANES spectroscopy. Comparison with DOS calculations for ferromagnetic Fe2P, indicates that the magnetism of Fe2P is mostly due to Fe(3g) sites. The magnetic moments calculated for Ni(3f) and Ni(3g) sites of ferromagnetic Ni2P approach zero, which is in accordance with the conclusion that Ni2P generally appears paramagnetic.

XAFS study of Ni (II)–aminovinylketone complexes

The functional properties of the active sites in a metalloproteins depend on coordination geometry of metal, the number and the nature of coordination ligands. The Ni K-edge XAFS spectra of novel nickel complexes as models for the MeN 2O 2(S 2) active site in metalloproteins were measured and analyzed. Theoretical analysis of the Ni K-edge XANES was performed using FDMNES code based on the finite difference method (FDM) to solve the Schrödinger equation beyond muffin-tin approximations and self-consistent full multiple-scattering approach (code FEFF8.2). It was found that the spectrum is almost totally formed by the octahedron of the nearest neighbor atoms around Ni ion in the II (Ni) complex. The III (Ni) complex active center exists in square-planar configuration with shorter distances.

Br K-Edge X-ray Absorption Near Edge Structure Analyses of Bromine Residue Carbon Compounds Using Full Multiple-Scattering Theory

A recent experimental result shows that Br K-edge X-ray absorption near edge structure (XANES) spectra for bromine residue compounds are sensitive to the heat treatment of host carbon compounds. In the present study we attempt to extract useful information on the geometric and electronic structures of these systems by applying a full multiple-scattering approach to XANES analyses. The residue compounds can be classified into three groups on the basis of XANES spectral features. Present detailed analyses provide us some useful information: Group 1, bromine residue compounds are prepared from host carbon compounds heat-treated at 2000–2800°C, where Br2 molecules exist in the interlayer space of graphite. The interlayer distance expands to about 7.8 Å around the Br2 molecules. For heat treatment at 1900°C, the observed spectra are well explained by the superposition of the spectrum for Group 1 and that for Group 2, with a ratio of 3:4. For Group 2 heat-treated at 1500–1750°C, the host carbon compounds includes Br- ions, but no information on the Br-–C correlation has been obtained. For Group 3 heat-treated at 1000°C, the host carbon compounds mainly includes HBr molecules parallel to small aromatic carbon molecules.

Lattice location of N in InxGa1-xAs1-yNy dilute nitrogen alloys

Dilute nitrides, in particular InxGa1-xAs1-yNy are very interesting alloys because of their actual and potential applications in the field of telecommunications and photovoltaics. A full knowledge of the local structure around each kind of atom, in particular the local ordering, is of fundamental importance because it is predicted to affect the electronics properties of these alloys. In this work we investigated the local environment of N and In by X-Ray absorption Fine Structure (XAFS). By exploiting the polarization dependency of XAFS we were able to demonstrate that N is fully substitutional to As. The lineshapes of the N K-edge XANES spectra are in fact identical, setting different grazing angles, which is a fingerprint of the cubic geometry of the N site. This information has been exploited to generate atomic clusters that very well simulate the experimental N K-edge XANES in the Full Multiple Scattering approach and to elaborate a multi-shell fit for XAFS data at the In K-edge. We concluded that N preferentially links In atoms, even if this short range ordering is by far weaker than the predicted one.

Ab initio calculations of the Ni K ‐edge XANES in Ni c Mg 1‐c O solid solutions

An interpretation of the Ni K-edge X-ray absorption near edge structure (XANES) in pure NiO and NicMg1-cO solid solutions is given based on three types of ab initio theoretical calculations: the full-multiple-scattering (FMS) approach, the finite difference method (FDM) and the full potential linear muffin-tin orbital (FP-LMTO) method within the LSDA+U approach. The first two methods differ in the cluster potential approximation: the spherically symmetric muffin-tin potential is used in the FMS approach, whereas a non-muffin-tin numerical potential makes the core of the FDM. We found that the FMS and FDM techniques allow rather accurate description of the experimental XANES signal especially above the absorption edge and produce close results, the FP-LMTO is more accurate close to the absorption edge. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic structure of MeS (Me = Ni,Co,Fe): x-ray absorption analysis

The sulfur K and L2,3 x-ray absorption near-edge structure (XANES) spectra of FeS, CoS and NiS having theNiAs-type crystal structure have been measured. Theoretical analysis of the experimentaldata has been conducted by the full multiple-scattering approach. Good agreement betweenexperimental and theoretical data is obtained for comparatively small clusters containing19–37 atoms. There is no significant difference between S K-edge absorption spectra for thestudied compounds calculated by G4XANES and FEFF8.2 packages. The electronicstructure of NiS and CoS has been studied by analysing the distribution of calculatedelectronic states of the sulfides.

Multiple scattering approach to the Pt L 2-edge XANES of CO adsorption on small Pt particles

A recent experimental result shows that Pt L-edge XANES spectra for Pt small cluster with ∼20 Pt atoms are sensitive to CO adsorption. In the present study we try to extract useful information about geometric and electronic structures of these cluster by applying a full multiple-scattering approach for the XANES analyses. Although IR analyses show that [CO] top/[CO] bridge is nearly constant for wide range of [CO] ad, it is difficult to estimate the ratio because of the large difference of the IR transition intensity for different adsorbed CO molecules. The present XANES analyses provide us useful new information [CO] top/[CO] bridge=1–2.

Full Multiple Scattering Analyses of Pt L-edge XANES for CO/pt Clusters.

A recent experimental result shows that Pt L-edge XANES spectra for Pt small cluster are sensitive to CO adsorption. In the present study we try to extract useful information about geometric and electronic structures of these cluster by applying a full multiple-scattering approach for the XANES analyses. Detailed analyses provide us with information that some extent of change transfer plays an important role for the CO adsorption. We also discuss the plausible adsorption site on the Pt clusters.

Full Multiple Scattering Approach to X-Ray Absorption near Edge Structure Spectra of Bromonaphthalene in Supercritical Xenon

Detailed X-ray absorption near edge structure (XANES) analyses are carried out in order to study the local structure around a Br atom of bromonaphthalene dissolved in supercritical fluid Xe by usage of full multiple scattering calculations. We find that about six Xe atoms surround the Br atom at the distance of about 5 Å, which is larger than the sum of the van der Waals radii (4.15 Å). Other important information is the relative thermal fluctuation (Debye-Waller factor) between Br and Xe, which is quite large, ∼0.8 Å. This large thermal fluctuation and clustering are essential characteristic features of supercritical systems.

X-ray absorption at the oxygen K edge in cubic f oxides examined using a full multiple-scattering approach

The O K-edge x-ray absorption near-edge-structure (XANES) spectra of UO2 and CeO2 are presented and interpreted. Using different-size clusters around the excited atom in the full multiple-scattering (MS) simulation, we are able to link the features present in the spectra of each oxide to its specific atomic arrangement and electronic structure. The structures at the edge originate from oxygen 2p states hybridized with f and d orbitals of the cation split by the cubic crystal field. All of the other features come from MS with the neighbouring shells of the central oxygen atoms.

Hard-XMCD and Magnetic EXAFS in the Multiple Scattering Approach

We simulated X-ray Magnetic Circular Dichroism and magnetic EXAFS at the L III edges of Rare Earths (RE) in some REFe 2 intermetallic alloys within the full multiple-scattering approach. The up and down potentials were obtained by means of a linear-muffin-tin orbital method. To test the influence of 4f electrons, whose very large role at the L edges of REis now well known, we chose Lu which has no 4f magnetic moment, Gd which has no 4f orbital momentum and Er which has a strong 4f orbital momentum. The edge and EXAFS regions are investigated. We compare the calculations with measurements.

Full Multiple-Scattering Approach to L3XANES of Germanium

The observed L 3 XANES spectra for crystalline and amorphous germanium are analysed by use of the full multiple scattering calculations. The prominent peaks found in the crystalline spectra are smeared out in the amorphous spectra. The difference of the observed XANES spectra is explained by the bond-angle fluctuation in the amorphous Ge where the Ge-Ge bond distance is fixed.

Characterization of iron oxides by x-ray absorption at the oxygen K edgeusing a full multiple-scattering approach

\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.

Electronic structure of SmFeO3: X-ray absorption fine structure analysis

Oxygen K edge and iron L2,3 edge X-ray absorption spectra of SmFeC3 have been measured and theoretical analysis of these data on the basis of full multiple-scattering formalism in real space has been made. Combined oxygen K edge and Fe L3 edge XANES is shown to be a useful tool for the study of RE orthoferrite electronic structure. The full multiple-scattering approach is found to be successful in analysing experimental oxygen K edge XANES in SmFeO3, while the single-scattering approximation is not valid in this case. Unoccupied oxygen p states interact with Fe 3d in a specific resonance manner.

Crystal-structure effects in the Ce L3-edge x-ray-absorption spectrum of CeO2: Multiple-scattering resonances and many-body final states.

Here we report the interpretation of all spectral features of Ce ${\mathit{L}}_{3}$ x-ray-absorption near-edge structure (XANES) of ${\mathrm{CeO}}_{2}$ over a 40 eV range. The local partial unoccupied density of states in the initial state and in the fully relaxed final state have been calculated by the full multiple-scattering approach. The wave function of the excited electron in the final states of the Ce ${\mathit{L}}_{3}$-edge XANES spectrum is found to be determined by the multiple-scattering processes in a large size cluster formed by at least 45 atoms. We predict, in good agreement with the experimental data, the crystal-field splitting of 5d states in the final state ${\mathrm{\ensuremath{\Delta}}}_{\mathit{f}}$=4.0\ifmmode\pm\else\textpm\fi{}0.2 eV and its variation from the initial to the final state. The many-body final states for the 2p\ensuremath{\rightarrow}5,\ensuremath{\varepsilon}d transition, arising from the configuration interaction due to mixing of Ce 4f and O 2p valence orbitals, have been calculated taking into account the crystal-field splitting of the 5,\ensuremath{\varepsilon}d states and they give a full explanation for the low-energy shoulder on the Ce ${\mathit{L}}_{3}$ x-ray-absorption spectrum white line that was the object of a long-standing discussion. The origin of the pre-edge peak at about 10 eV below the white line maximum is explained as due to transitions at the bottom of the conduction band.

Two-electron excitations and one-electron multiple-scattering resonances in the x-ray absorption of solid neon.

The local and partial density of states of the conduction band of solid neon in a large energy interval has been extracted from the analysis of the K-edge x-ray-absorption near-edge structure (XANES) spectrum by using a full multiple-scattering method. The absorption cross section for one-electron transitions from the Ne 1s level to the conduction-band final states in the solid Ne is predicted by first principles. The absorption cross section is described as due to an atomic factor and to a structure factor determined by the local and partial density of states for a large cluster formed by about 80 atoms. The experimental spectrum is obtained by describing the final state in the fully relaxed potential in the presence of the core hole. The exciton peak at threshold is shown to be due to the lowest unoccupied valence state pushed down in the bound-state region by the core hole. In the 100-eV XANES energy range the single-scattering approximation fails to predict the XANES of solid Ne indicating that the full multiple-scattering approach is required. The present analysis allows us to identify strong two-electron excitations 1${\mathit{s}}^{2}$2${\mathit{s}}^{2}$2${\mathit{p}}^{6}$\ensuremath{\rightarrow}1${\mathit{s}}^{1}$2${\mathit{s}}^{2}$2${\mathit{p}}^{5}$${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{i}}$${\mathit{p}}^{2}$ in the energy range of 30 eV above the K edge, involving monopole 2p\ensuremath{\rightarrow}\ensuremath{\varepsilon}p transitions to the lowest valence states at energy \ensuremath{\varepsilon}. The intensity of the two-electron excitations is found to be so strong that it should be considered in standard extended x-ray-absorption fine-structure--XANES data analysis for local structure investigation.

Full Multiple-Scattering Approach to K-Edge XANES of SeTe Mixture

The Se K-edge XANES spectra of SeTe mixture are studied by use of a full multiple-scattering approach. We calculate the Se K-edge XANES of Se50Te50 mixture crystal, in particular we study various atomic arrangements in the spiral chains. As a result of the comparison between these calculated and experimental spectra, we can grasp an image of the substitution in these mixtures; the homogeneously mixed model, –SeTeSeTe– is ruled out.