Showing papers on "XANES published in 1984"

A XANES investigation of the bonding of divalent lead in solids

Abstract: XANES (X‐ray absorption near‐edge structure) spectra of divalent lead compounds have been studied at both the Pb LIII and LI edges using synchrotron x rays from CHESS (Cornell High Energy Synchrotron Source). The combined LIII and LI near‐edge spectral features strongly support a sp3d2 hybridization scheme in covalently bonded yellow PbO. The absorption features associated with the spectra of ionic halides (PbF2, PbCl2, and PbI2) and the oxysalts [Pb(NO3)2, PbSO4, and PbCrO4] have been shown to be consistent with crystal field splittings of the final states using group theoretical arguments. The XANES spectra of a series of PbO–PbCl2 glasses have also been studied to elucidate the nature of bonding and confirm the local structure derived from x‐ray diffraction experiments. Shape resonance features associated with LIII and LI edge XANES have also been discussed. This investigation illustrates the novel use of XANES spectroscopy in studying the bonding of nontransition metal ions.

Structural characterization by X-ray absorption spectroscopy (EXAFS/XANES) of the vanadium chemical environment in Boscan asphaltenes

Abstract: X-Ray absorption spectroscopies, EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near-edge structure), were used for probing the structural and chemical environment of vanadium in two different asphaltenic fractions of a Boscan crude oil. Although the level of porphyrins detectable by u.v.–visible spectrophotometry was substantially reduced (i.e. <15% from the 4 000 p.p.m. of metal) due to successive solvent extractions, the EXAFS spectra of these fractions were still largely dominated by the typical pattern of vanadyl porphyrins, whereas the intense prepeak observed in the XANES spectra also clearly supported the presence of quite large amounts of porphyrins in these fractions. We thus suggest that the u.v.–visible spectrophotometric analyses tend systematically to underestimate the porphyrinic content in these heavy fractions, probably due to some hardly detectable microheterogeneity of the test solutions. E.s.r. spectra are produced which clearly support these various conclusions. Much more elaborate separation procedures are required in order to isolate and identify any ‘non-porphyrinic vanadium fraction’ from Boscan crudes.

Distance Dependence of Continuum and Bound State of Excitonic Resonances in X-Ray Absorption Near Edge Structure (XANES)

Abstract: The rising interest in XANES is due to the experimental evidence that information on coordination geometry and bonding angles, not given by EXAFS, can indeed be extracted from XANES [1,2]. In this paper I would like to point out that certain features of XANES, being sensitive to interatomic distances, can also be exploited to determine first coordination bond lengths to a few percent accuracy (5% or less) [3]. This aspect of XANES is particularly useful whenever EXAFS modulations are too weak to be measured, as in the case of low 7 backscatterers. For the sake of concreteness, I shall limit myself to K-edge absorption spectra of simple molecules containing C-C, C-N, C-O type of bonds and of two series of Mn and V oxides (see Tables I and II). These cases are general enough to serve as an illustration of other cases.

An absorption edge and extended X-ray absorption fine structure study of a dispersed manganese dioxide oxidation catalyst

Abstract: Variously treated samples of a low-cost MnO2–SiO2 catalyst designed in the first instance for the complete vapour-phase oxidation of hydrocarbon and other vapours in pollution-control applications have been examined. The EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near-edge structure) characteristics of samples deliberately activated, deactivated or thermally degraded, i.e. respectively oxidised, reduced or destroyed, are compared. The oxidation number or effective atomic charge of the manganese ion in these samples is identified, as are also changes in the Mn—O bond distances. For comparison and reference, analogous data from bulk manganese oxides of known stoichiometry were also measured and are briefly considered.

Ti-site geometry in metamict, annealed and synthetic complex TiNbTa oxides by X-ray absorption spectroscopy

Abstract: Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopy has been used to determine the nearest neighbor environment of Ti in metamict AB2O6-type complex oxides (euxenite and aeschynite structure types) in order to evaluate the effect of alpha-recoil damage of these structures. Comparison of the EXAFS/XANES data for metamict samples with data for annealed and crystalline samples suggests minor changes in the first coordination sphere, TiO, (a slight decrease in coordination number and bond lengths, and increased distortion of the octahedral coordination polyhedron), but major disruption of the second coordination sphere, TiTi, for the material in the metamict state. These data suggest a mechanism for the transition from the crystalline to the metamict state in which tilting of cation coordination polyhedra is the principal effect of damage caused by alpha-recoil events.

EXAFS and XANES studies of Zn2+ and Rb+ neutralized perfluorinated ionomers

Abstract: Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) experiments have been carried out to probe the Zn2+ and Rb+ environment in perfluorinated ionomers. The cation environment has been determined for these ionomers in their dry, hydrated and n-amyl alcohol swollen state. It was found that a well ordered, crystalline-like nearest-neighbor oxygen shell predominates in the zinc neutralized perfluorinated ionomers. Unlike the zinc ionomers, the Rb+ neutralized ionomers show no discernible peaks in the radial structure function indicating that the rubidium environment is highly disordered. Coordination of the hydroxyl groups of namyl alcohol to cations was suggested by EXAFS analysis. XANES analysis was useful in corroborating the EXAFS information and in providing information about the ionic character of the nearest-neighbor bonding.

Application of EXAFS and XANES Spectroscopy to Problems in Mineralogy and Geochemistry

Abstract: Mineralogy and geochemistry have long used conventional diffraction and spectroscopic methods to structurally characterize natural crystalline phases [1] and silicate glasses [2]. However, the relatively new spectroscopic methods made available by high intensity synchrotron sources have been utilized only recently by a few geochemists and mineralogists [3,4]. In particular, EXAFS and XANES spectroscopies are capable of yielding unique structural information about most elements of geochemical importance (most of the periodic table), whether they occur as trace (0.1 Wt.%), minor (0.1–1.0 Wt.%), or major elements in crystalline or amorphous solids, aqueous electrolyte solutions, or organic matter such as coal. This paper is a brief survey of EXAFS and XANES results we have obtained recently on a variety of earth materials intended to illustrate the utility of these methods in addressing problems of geologic importance. The topics considered include: Structural characterization of local environments of Al and Na in glasses in the system Na2O-Al2O3-SiO2; the study of Fe site partitioning and Fe/Mg clustering in minerals; the characterization of transition metal complexes (Fe and Zn) in aqueous chloride solutions; and the study of structural changes accompanying amorphitization of mineral phases such as zircon (ZrSiO4).

Surface Crystallography by Means of SEXAFS and NEXAFS

Abstract: The availability of high brightness (flux/unit area) monochromatized synchrotron radiation [10.1] has allowed the development of new experimental techniques which tune into specific surface atoms and probe their structural environment. Two such techniques, the subject of this chapter, are the surface extended X-ray absorption fine structure (SEXAFS) and near-edge X-ray absorption fine structure (NEXAFS) techniques. Both measure the X-ray absorption by a specific atom on the surface which is distinguished from other atoms by one of its main absorption edges (usually K or L edge).

K-Edge XANES Spectra of GaAs and ZnSe

Abstract: X-ray absorption near edge structure (XANES) spectra for crystalline GaAs and ZnSe are measured for each K-edge region. Full multiple scattering calculations are also performed. The information on the charge distribution around the X-ray absorbed atom can be obtained if we compare the experimental XANES spectra with full multiple scattering calculations. In these systems core hole potential is important to analyse the spectra.

EXAFS and XANES Studies of Superionic Cu 2-x Se

Abstract: The measurement of the EXAFS spectrum of superionic Cu 1.8 Se has been carefully carried out. The local structures around the Cu and Se atoms are determined. The Cu atoms around the tetrahedral site of Se atoms are detected but those around the octahedral site are not found. The XANES spectrum of Cu 1.8 Se was also measured and compared with that of Cu 2 Se.

XANES: One Electron Multiple Scattering Resonances and Splitting of Final State Configurations at Threshold

Abstract: The energy range of the X-ray absorption near edge structure (XANES) and the definition of the X-ray absorption threshold have been recently object of discussion. Here a definition of the absorption threshold is proposed and the energy range of XANES is discussed. The different physical processes present in the XANES range will be resumed and the role of multiple scattering resonances in the angular resolved Fe XANES of myoglobin single crystal will be shown.

Effect of Intermediate Range Order on the Lineshape of the Ge K Edge

Abstract: EXAFS investigations on a-Ge [1–3] deposited at different temperatures pointed to the nucleation of ordered grains in the amorphous matrix, whose size increases with temperature. This process was revealed by important modifications of the EXAFS spectrum, Χ(k), and consequently of its Fourier transform. Besides these modifications in the EXAFS part of the spectrum, significant changes in the shape of the near edge structure (XANES) occur which should be correlated to the structural evolution of the system. It is the purpose of this paper to show that this is indeed the case and that a careful analysis of the near edge structure (associated with a calculation of the absorption cross section) can elucidate the relation between fluctuations of bond distances and edge shape.

Near Edge X-Ray Absorption Fine Structure Studies of Chemisorbed Molecules

Abstract: The K-edge photoabsorption spectrum of low Z atoms (B,C,N,O,F) in molecules chemisorbed on transition metal surfaces is dominated by strong intramolecular resonances in the energy region within ∼40eV from the excitation threshold. We will refer to this part of the spectrum as the Near Edge X-ray Absorption Fine Structure (NEXAFS). For molecules composed of low Z atoms (B,C,N,O,F), two kinds of resonances are observed which are generally referred to as π and σ resonances on the basis of the symmetry of the final states involved in the excitation. Such near edge structures arise from scattering processes within the intramolecular potential, which is determined by the atomic cores and the valence charge distribution of the molecule, and are to a large extent decoupled from the extramolecular environment [1]. This fact renders the NEXAFS spectrum of a chemisorbed molecule a powerful local probe for monitoring the modifications of the intramolecular bond induced by the reaction with the metal surface. In spite of the fact that a quantitative interpretation of the NEXAFS spectrum requires detailed calculations where localization and correlation effects are taken into proper account [1,2], we will show that important geometric and electronic structure information, like the orientation of the molecule with respect to the surface, bond-length changes upon chemisorption and molecular orbital rehybridization, can be directly linked with general properties of the near edge molecular resonances.

XANES of Palladium Rare Earth Intermetallics (RPd3): Determination of Hybridization and Mixing of 4f-Orbitals

Abstract: The single site promotional model of heavy rare earth valence fluctuating compounds has been supported by XPS and XANES experiments [1,2]. Light rare earth compounds exibith anomalous electronic properties due to hybridization of the 4f-orbitals of the rare earth with the conduction band.

O2 and CO Bonding Geometry in Heme-Proteins in Solution Investigated by XANES

Abstract: The bonding angle of oxygen and carbonmonoxy molecules in oxygen carrier hemoproteins is a key parameter to describe the bond strength of the diatomic molecules to iron atom and therefore to understand the mechanism of reversible bonding and releasing of oxygen in hemoglobin. In spite of the large number of studies of haemoglobin the determination of oxygen bonding angle in the proteins in solution escapes the available experimental methods. A different bonding angle of oxygen in oxy-hemoglcbin (HbO2) and in oxy-myoglobin (MbO2) single crystals has been recently reported [1]. The different bonding angle in the two hemoproteins which have different biological roles, transport and storage of oxygen molecules respectively, but similar local structure at the iron site has renewed the interest in this problem. We report the first application of the XANES (X-ray Absorption Near Edge Structure) spectroscopy to obtain information on the oxygen bonding geometry in the protein in solution (close to the “in vivo” situation) which cannot be studied by diffraction methods. The appealing aspect of XANES to study this problem is that the multiple scattering resonances of the photoelectron emitted at the iron site in the 10–50 eV energy range depend on the relative atomic positions of neighbour atoms and not only on the first order radial distribution function like EXAFS (Extended X-ray Absorption Fine Structure), therefore it is a direct structural probe of bonding angles [2–4].

XANES and EXAFS Measurements on Pd2Si: One-Electron or Many Body Effects?

Abstract: X-ray absorption spectroscopy has been used to investigate near L-edge structures of Pd in pure Pd and bulk Pd2Si and PdSi silicides (1)

Reconciliation of Valency Ambiguity in V2TiO5 Using XANES Spectroscopy

Abstract: The structure of V2TiO 5 involves a distribution of the two metal species between two crystallographic sites. The sites between the shear planes are occupied solely by vanadium as V m, while the shear-plane sites are occupied by both metal atom species, vanadium and titanium. An analysis of the metal-oxygen distances could not distinguish between the two reasonable alternatives for the valency distribution, viz VH~/Ti w or V~V/Ti m. By examining the XANES spectra of the title compound at both vanadium and titanium K edges the valency distribution can be confidently assigned as VHI/Ti ~v. The usefulness of XANES spectroscopy for solving similar problems involving valency ambiguity is evident.

X-Ray Lβ2,15 Emission Spectrum of Ru in Ru(NH3)6Cl3

Abstract: One of the broader applications of synchrotron radiation has been to EXAFS studies for material structure determination, i. e., for an analysis of x-ray absorption over an extended energy region beyond a core ionization limit. Studies of the near edge structure (XANES) give a different type of information, characteristic of the local symmetry and electronic configuration of the absorbing atom. This type of information is reflected also in the x-ray emission spectra, in particular for transitions involving the valence levels. Examination of the near edge absorption or the emission spectrum does not require an instrument capable of scanning a wide energy range with high counting statistics, as does EXAFS; the needs are rather for good resolution and a reliable calibration of the energy scale.

Surface Structure Determinations From Near Edge X-Ray Absorption Fine Structure Studies

Abstract: Surface structure determinations from near edge x -ray absorption fine structure studiesD. Norman and P.J. DurhamScience and Engineering Research Council, Daresbury Laboratory, Daresbury,Warrington WA4 4AD, U.K.AbstractWe discuss the x -ray absorption near edge structure (XANES) above the K -edges of lightelements in atomic and molecular adsorption on single -crystal surfaces. We concentrate onthe systems O on Ni(100) and CO on Ni(100), and compare experimental data with the resultsof multiple- scattering calculations. The role of multiple- scattering and the use of lesserapproximations is briefly discussed. The atomic adsorption of oxygen provides a reasonablysensitive test of adsorption site, but the near edge structure is influenced by as many as30 neighbour atoms. By contrast, for the molecular adsorption of CO, the XANES is dominatedby intra- molecular scattering with negligible contributions from the substrate atoms.Monitoring the XANES for molecular adsorbates is shown to provide an extremely accuratemeasure of changes in intra- molecular distances upon chemisorption: an accuracy of 0.01 A isreadily achieved, far better than the results of any other surface structural method, andsufficient for the deduction of chemical trends in bonding.Introduction and theoryThe mechanism for the absorption of photons in solids is straightforward: when the energyof an x -ray exceeds the binding energy of a core level, the photon can be absorbed by excit-ing an electron to an outgoing photoelectron wave. The matrix element for this transitionis modified by the presence of neighbouring atoms which may scatter the photoelectron wave,leading to modulations in the absorption coefficient. Far above the absorption edge, suchscattering is weak, facilitating the analysis of this extended x -ray absorption fine struc-ture (EXAFS) by single- scattering theory. However, within about 50 eV of an absorptionedge, scattering becomes stronger and mean -free -paths of the photoelectrons are longer,leading to sizeable multiple- scattering contributions and a complicated theory. But thisx -ray absorption near edge structure (XANES, otherwise called NEXAFS, near -edge x -ray ab-sorption fine structure) exhibits the largest modulations in the absorption coefficient, andmay - particularly for the light elements with K edges in the soft x -ray region - be theonly structure visible in a spectrum.A computational scheme has been developed at Daresbury which allows the analysis of theXANES regime'. The theory has been described before2, and some aspects of it discussed indetail elsewhere3, so here we merely summarise its salient points. The theory includes fullmultiple scattering, and is based on a cluster method. The calculation proceeds by dividingthe cluster into shells of atoms (each of whose scattering properties are described by a setof phase- shifts), and the multiple scattering equations are solved within each shell inturn, with the final step being calculation of the multiple scattering between shells andassembly of the whole cluster. The size of the scattering matrices is reduced by use ofsymmetries (mirror or rotational) of the cluster, as we pick out, in the course of themultiple scattering calculation, only those components of the angular momentum expansionswhich can be coupled by a Hamiltonian possessing these symmetry elements. The reflectionmatrix thus calculated, when combined with an atomic matrix element linking the core andexited electron states, gives the XANES cross -section exactly in one -electron theory.Many -body decay processes, which limit the lifetime of the core hole and excited photo-electron, are included in the calculation as a complex (absorptive) potential whose effectis to broaden spectral features on the appropriate scale.This paper summarises some of our results from using this theory to calculate the XANESof adsorbates on surfaces.Atomic adsorption: oxygen on Ni(100)It is well established4 that oxygen adsorbs atomically on Ni(100), and that withincreasing 0 coverage different chemisorption geometries are produced. The oxidation ischaracterised by the consecutive formation of a p(2x2), a c(2x2) and a (1x1) Ni0 low energyelectron diffraction (LEED) pattern. There has recently been some controversy over thestructure of the p(2x2) and c(2x2) phases5.Figure 1 displays the experimental spectra for the c(2x2) phase of oxygen chemisorbed onNi(100), collected for two different angles of incidence (with the electric polarisationvector nearly normal to the surface, and with E in the surface plane), compared with our

Structural characterization by x‐ray absorption spectroscopy (exafs/xanes) of the vanadium chemical environment in boscan asphaltenes

Abstract: X-Ray absorption spectroscopies, EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near-edge structure), were used for probing the structural and chemical environment of vanadium in two different asphaltenic fractions of a Boscan crude oil. Although the level of porphyrins detectable by u.v.–visible spectrophotometry was substantially reduced (i.e. <15% from the 4 000 p.p.m. of metal) due to successive solvent extractions, the EXAFS spectra of these fractions were still largely dominated by the typical pattern of vanadyl porphyrins, whereas the intense prepeak observed in the XANES spectra also clearly supported the presence of quite large amounts of porphyrins in these fractions. We thus suggest that the u.v.–visible spectrophotometric analyses tend systematically to underestimate the porphyrinic content in these heavy fractions, probably due to some hardly detectable microheterogeneity of the test solutions. E.s.r. spectra are produced which clearly support these various conclusions. Much more elaborate separation procedures are required in order to isolate and identify any ‘non-porphyrinic vanadium fraction’ from Boscan crudes.

A study of Co/Mo/Al2O3 hydrodesulphurization catalysts and related model compounds byxps and x-ray absorption spectroscopy (xanes andexafs)

Abstract: A detailed investigation of sulphided Co/Mo/Al2O3 catalysts, their oxide precursors and several model oxides and sulphides of cobalt and molybdenum has been carried out using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy (xanes andexafs). Octahedrally coordinated Co(II) and Mo(IV) are shown to be present in a sulphidic environment on the surfaces of these catalysts. The surface species contain an excess of sulphur, probably involving disulphide linkages. The surface compositions of the catalysts examined conform to the general formula Co11 Mo 2n IV (2n + 3)S 2 2− (2n -2)S2−.