Showing papers on "Extended X-ray absorption fine structure published in 1992"

Full correction of the self-absorption in soft-fluorescence extended x-ray-absorption fine structure.

Abstract: The amplitude of the extended x-ray-absorption fine structure of concentrated samples measured in the fluorescence mode (FLEXAFS) as well as the overall shape of the fluorescence-yield spectra strongly depend on the detection geometry through the self-absorption effect. In these cases, a conventional EXAFS analysis can lead to systematic errors in the determination of physical parameters. We studied the distortions in the FLEXAFS spectra through the self-absorption effect measuring the FLEXAFS of a NiO single crystal above the oxygen K edge for various detection geometries. We show that knowing the stoichiometry of the sample we can fully correct for the self-absorption effect using a simple theory and obtain the correct, geometry-independent oxygen EXAFS of NiO. The correction procedure presented here for the prototype system of NiO is generally applicable and should be the first step in the analysis of FLEXAFS data of concentrated samples. We calculate the information depth of the fluorescence detection as a function of the experimental geometry. The knowledge of the self-absorption in relationship to the information depth allows the determination of the optimum experimental setup.

X-ray absorption spectroscopic study of the sorption of Cr(III) at the oxide-water interface

Abstract: The oxidation of Cr(III) to Cr(VI) at Mn(IV) and Mn(III) oxide/water interfaces was investigated by X-ray absorption spectroscopy. The sample [Cr(VI)]/[Cr(VI) + Cr(III)] ratios within the solid-phase samples were accurately determined by two independent methods: the intensities of preabsorption edge spectra and the extended X-ray absorption fine-structure (EXAFS) contribution of the Cr-O atomic pair. Modifications of the local structure around chromium atoms along the redox process were followed by EXAFS. The oxidation process was elucidated at the birnessite (Na4Mn14O27 · 9H2O) surface by means of a kinetic study of the Cr sorption process at pH 4. After a 30-s reaction time, while chromate ions are still few (≈5%), sorbed chromium octahedra are bonded to birnessite by sharing five or six edges with MnO6 octahedra. Ninety seconds later, the chromate concentration is high, and the Mn(IV)-O-Cr(III) bridges are no longer detected. According to these results the mechanism of the oxidation can be depicted as follows: (1) Cr(III) aqua ions diffuse toward Mn(IV) vacancies present in the sheet of MnO6 octahedra; (2) the coupled Cr(III) oxidation/Mn(IV) reduction occurs; (3) Cr(VI) ions are released into the solution. Experimental data suggest that the oxidation process on Mn(III) oxides is similar to that depicted for birnessite. This study provides the first evidence of a double solid-state diffusion, first toward and then backward from the sorbent. These results illustrate how the structure of the sorbent, through the nature of the active site, may control the speciation of elements in aqueous media.

Local structure of 1.54‐μm‐luminescence Er3+ implanted in Si

Abstract: Extended x‐ray absorption fine structure measurements from Er‐implanted Czochralski‐grown Si samples, which exhibit strong luminescence at 1.54 μm, reveal a local sixfold coordination around Er−not of Si−but of oxygen atoms at an average distance of 2.25 A. By contrast, similar concentrations of Er implanted in high purity float‐zone Si samples, which are essentially optically inactive, show that Er is coordinated to 12 Si atoms at a mean distance of 3.00 A.

Origin of the electrocatalytic properties for oxygen reduction of some heat-treated polyacrylonitrile and phthalocyanine cobalt compounds adsorbed on carbon black as probed by electrochemistry and x-ray absorption spectroscopy

Abstract: Electrochemical and X-ray absorption techniques have been used to determine the influence of heat treatment in electrocatalytic activity for O 2 reduction for two cobalt catalysts. The catalysts are cobalt phthalocyanine (catalyst 1) and polyacrylonitrile +cobalt acetate (catalyst 2) adsorbed on carbon black and heat treated at several temperatures. A maximum for the catalytic activity was obtained for PcCo at 850 o C and for the PAN+Co catalyst at 950 o C with subsequent decrease. The results obtained by XANES and EXAFS data clearly show that metallic cobalt aggregates with different size are synthetized in the range of increased activity

Structure of self-organizing organic films: A near edge x-ray absorption fine structure investigation of thiol layers adsorbed on gold

Abstract: Self‐assembled films of thiols on polycrystalline Au substrates have been investigated with near edge x‐ray absorption fine structure. From the angular dependence of two different resonances the orientation of the molecular axis of the alkyl chains with respect to the surface normal is determined and compared to results obtained by other techniques. By comparing data for thiol films and LB films of Cd and Ca arachidate it is found that thiol films are considerably better ordered.

Structure of polymer-protected palladium/platinum bimetallic clusters at the oxidized state. Extended X-ray absorption fine structure analysis

Abstract: The mild oxidation and reduction processes of the Pd/Pt bimetallic clusters protected by poly(N-vinyl-2-pyrrolidone) were investigated with an extended X-ray absorption fine structure (EXAFS) technique. From the intensity of the EXAFS spectrum due to the Pd-O bond, each Pd atom on the surface of the Pd/Pt(4/1) cluster is estimated to be surrounded by the 0.21-2.3 atoms of oxygen, depending on the extent of the oxidation. Depending on the contact time with molecular oxygen, the Pd-O bond of the bimetallic clusters has a different coordination distance from 1.96 to 2.17 A, about 0.15 A longer than the first coordination distance of palladium oxide (2.03 A)

The state of excessively lon-exchanged copper in mordenite: formation of tetragonal hydroxy-bridged copper ion

Abstract: Copper ions are exchanged in mordenite in amounts in excess of the value expected from stoichiometric considerations in the process of repeated ion-exchange of sodium ions in mordenite with copper ions. This is not the case for nickel and calcium ions used as exchanger ions. A series of experiments was designed to elucidate the state of excessively ion-exchanged copper in mordenite. DRS and EPR analyses together with XANES spectra revealed that the oxidation state of copper ion in mordenite is divalent in each exchange stage. IR, EPR, XANES and EXAFS studies provided evidence for the existence of a tetragonal hydroxy-bridged polymer of copper ions in excessively ion-exchanged mordenite; the Cu—O bond length is 1.97 A and the Cu–Cu distance is 3.05 A. This polymer species exhibited IR absorption bands near 3350 and 930 cm–1 for OH stretching and MOH bending vibrations of bridged species, respectively. The difference in exchange behaviour of divalent metal ions is interpreted in terms of the difference in the magnitude of their hydrolysis constants.

Structural Study of Tin‐Doped Indium Oxide Thin Films Using X‐Ray Absorption Spectroscopy and X‐Ray Diffraction I . Description of the Indium Site

Abstract: Thin films of tin‐doped , with different tin contents produced by a powder pyrolysis technique have been investigated by x‐ray absorption spectroscopy and x‐ray diffraction. At the indium K‐edge, extended x‐ray absorption fine structure (EXAFS) investigations reveal that the indium atomic environment is modified by the doping. Even at low tin concentrations, the first oxygen polyhedron and the metallic In‐In coordination shells are disordered. This disorder increases with increasing tin content, and, for a large tin content (39% Sn), the In‐O shell is locally similar to that of the hexagonal high pressure phase of , that is also known to be stabilized at atmospheric pressure by the insertion of small foreign cation (such as Sn4+) in the indium oxide structure. The x‐ray diffraction experiments also support the idea of a disordered network; the x‐ray reflections decrease in magnitude but remain sharp even for a high tin concentration. This indicates that the structural disorder observed by EXAFS only induces a damping of the x‐ray reflections (the network is still ordered within large domains). The following modifications of the host network were observed; an increase in the lattice parameter, a decrease in the size of coherence of the diffraction domains with increasing the tin content (from 1200 A to 700 A), and a preferential orientation, which disappears for the high dopant concentrations.

Analysis of in-situ spectral absorption meter data

Abstract: In situ absorption data collected with a reflective-tube absorption meter are presented. Various procedures for correcting the data for scattering error and for extracting chlorophyll absorption from the raw signal are explored. Based on our knowledge of the distribution of particle types and on measured backscattering, the scattering correction as a function of total scattering is found to vary significantly with depth. However, absorption in the near infrared is shown to be highly correlated with backscattering. We thus postulate that this signal is mainly due to the scattering error and possibly to absorption by dissolved substances and particles of a detrital nature. Thus, the infrared signal seems to provide a good correction for the measured a(676) to obtain chloropigment absorption. Indeed, a(676) - a(750) (corrected for water absorption and 750 temperature dependence) was found to correlate strongly with fluorescence. However, a(750) was found to be highly temperature dependent, so a(712) was chosen for this purpose in future measurements. An examination of optical microstructure in East Sound, Orcas Island, Washington showed numerous peaks with vertical dimensions of the order of tens of centimeters. These peaks may contain the majority of the biomass in the system. The relative magnitude of the a(676) - a(712) and the a(712) signals varied greatly from one peak to another and systematically with depth, presumably reflecting the nature and physiological states of the populations in the various peaks.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Band‐to‐band optical absorption in narrow‐gap Hg1−xCdxTe semiconductors

Abstract: The infrared absorption spectra have been measured at temperatures from 4.2 to 300 K for thin bulk samples of Hg1−xCdxTe with compositions, x, from 0.170 to 0.443 with thicknesses smaller than 10 μm. The spectral range of the investigations includes the exponential absorption edges and intrinsic absorption spectra determined by electron transitions from the valence band to the conduction band. From the measured absorption spectra, the energy‐band gap was determined by the photon energy at the absorption transition point between the Urbach exponential region and the Kane region. The data were fitted by a calculation of the intrinsic optical transition using Kane’s theory. The dependence of the energy‐band gap on composition and temperature was determined in the alloy composition range from x=0 to 0.443 and for x=1, for temperatures from T=4.2 to 300 K. The absorption coefficient, αg, at the band‐gap energy versus composition and temperature, and the exponential rule satisfied by the absorption edge are als...

Determination of the dispersive correction f'(E) to the atomic form factor from X‐ray reflection

Abstract: It is shown in this paper that the reflectivity of X-rays at smooth and fiat surfaces gives the dispersive correction f′(E) to the atomic form factor with an accuracy comparable to that obtained by X-ray interferometry. A detailed set of values of f′(E) in the energy range 7–27 keV is given for Ni, Cu, CuO, Ta, LiTaO3, Pt and Au, together with the corresponding linear absorption coefficients μ/ρ. Whenever comparison is possible the values of f′(E) agree very well with those obtained by interferometry or by Kramers–Kronig transformation. Data calculated according to Cromer & Liberman [J. Chem. Phys. (1970). 53, 1891–1898] agree well with our data far from absorption edges. At the edges there are substantial differences because the calculations do not take into account the structure of the edges, their chemical shift in compounds and the EXAFS structure above the edges. Below absorption edges the values of f′(E) for metals and their oxides are equal, provided the chemical shift in the position of the edges is taken care of. This feature is interesting in anomalous scattering experiments, where the variation off' with energy is used to vary the scattering contrast of a given atomic species. Once f′(E) is known, X-ray reflectivity measurements can be used to determine the density and the thickness of layers on fiat substrates. In addition, the roughness of the air-layer and layer-substrate interfaces have been determined with high precision in the metals and oxides mentioned above.

Monte Carlo modeling of the performance of a reflective tube absorption meter

Abstract: A Monte Carlo model of the behavior of photons in a reflective tube absorption meter has been developed and used to investigate how well such a meter can measure the absorption coefficient in waters of different optical types. The apparent, i.e., measured, absorption coefficient (alpha(m)) is always greater than the true absorption coefficient (alpha). The ratio alpha(m)/alpha increases linearly with the ratio of scattering to absorption (b/alpha) at a rate that depends on the scattering phase function of the water. The excess attenuation is mainly due to the failure of forward-scattered photons to undergo reflection at the cylinder wall. To achieve the highest accuracy possible with the meter, one must maximize the reflectivity and detector acceptance angle. Performance is also improved by increasing the cylinder diameter.

Structure of the αFe x Al 1:x OOH solid solution

Abstract: The three-dimensional local organisation around Fe atoms in a natural diaspore (αAl0.9955Fe0.0045OOH) has been investigated by angular measurements of X-ray absorption spectra. It is demonstrated that in a single crystal of diaspore, the absorption cross-section exhibits the special case of trichroism where three independent measurements are needed to determine the absorption cross-section for any direction of polarization. Extended X-ray absorption fine structure (EXAFS) spectra were thus recorded at the following orientations of the polarization vector: $$$$ , $$$$ , and $$$$ . The incoming white beam was monochromatized using two Si(331) crystals, which deliver at the Fe K-edge a completely linearly polarized X-ray beam. The reliability of our measurements was checked by comparing the isotropic EXAFS spectrum calculated from the three orthogonal measurements to the one of the diaspore powder recorded at the magic angle. It is shown that Fe3+ ions are not randomly distributed within the diaspore framework. Furthermore, only part of the Fe3+ ions substitutes Al, the others being located in the channels of the structure. The 3D local structure of the Fe domains has been modeled assuming hematite-like clusters of three Fe octahedra topotactically grafted to aluminous chains. These Fe clusters are thought to represent ancient multinuclear Fe surface complexes having formed at the time of the diaspore growth, and being sealed in its bulk structure. The whole crystal of diaspore is then thought to have kept the memory of the heterogeneous nucleation mechanism of these hematite nuclei. In addition to the well-known examples of atom segregations, ion vacancies, and intergrowths of discrete phases, this new type of crystal defect represents another evidence of non-equilibrium crystallization process under the thermodynamic and kinetic conditions that prevail at the earth's surface.

An EXAFS study on the so-called "Co-Mo-S" phase in Co/C and CoMo/c, compared with a Mössbauer emission spectroscopy study

Abstract: EXAFS was used in this paper to study 4 sulfided catalysts that have the same structure as their {sup 57}Co counterparts characterized by Mossbauer emission spectroscopy. The {open_quotes}Co-Mo-S{close_quotes} phase in Co/C is similar to CoMo/C due to a very highly dispersed Co species. Without Mo, the sulfidic Co results in a Co{sub 9}S{sub 8} phase during sulfidation at 673K, Mo prevents Co sintering in CoMo/C. 37 refs., 6 figs., 2 tabs.

Bond-length relaxation in crystalline Si1-xGex alloys: An extended x-ray-absorption fine-structure study

Abstract: Extended x-ray-absorption fine-structure spectra for crystalline ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ alloys, measured at the K edge of Ge at room temperature, are analyzed with a curve-fitting method based on the spherical-wave approximation. The Ge-Ge and Ge-Si bond lengths, coordination numbers of Ge and Si atoms around a Ge atom, and Debye-Waller factors of Ge and Si atoms are obtained. It is shown that Ge-Ge and Ge-Si bonds relax completely, for all Ge concentrations of their study, while the lattice constant varies monotonically, following Vegard's law. As noted by Bragg and later by Pauling and Huggins, the Ge-Ge and Ge-Si bond lengths are close to the sum of their constituent-element atomic radii: nearly 2.45 \AA{} for Ge-Ge bonds and 2.40 \AA{} for Ge-Si bonds. A study on the coordination around a Ge atom in the alloys revealed that Ge and Si atoms mix randomly throughout the compositional range studied.

An EXAFS study of the metallofullerene YC82: is the yttrium inside the cage?

Abstract: A sample, determined by time-of-flight mass spectroscopy (TOFMS) to consist of YC 82 as the major metal-fullerene complex, was analyzed by X-ray absorption spectroscopy. The Y is found to have 7±1 near-neighbor C atoms at 2.35±0.02 Aand an Y neighbor at 4.05±0.05 A. The unequivocal observation of an Y-Y interaction is unexpected, since the mass spectral data show no indication of Y 2 C n as a major component of the sample. We believe that the combined TOFMS and extended X-ray absorption fine structure (EXAFS) results are not consistent with models that place the metal ion inside the fullerene cage

The correlation of the 7.6 eV optical absorption band in pure fused silicon dioxide with twofold-coordinated silicon

Abstract: The optical absorption band at 7.6 eV, which appears in oxygen deficient pure silica, does not correlate with any ESR signal in non-irradiated samples. Longlasting illumination at 80 K in the range of its absorption leads to an increase of the absorption band at 5 eV. Subsequent heating to 290 K restores the initial absorption. These data can be explained as photodissociation and thermal recreation of a complex defect containing a twofold-coordinated silicon defect. This complex defect is responsible for the 7.6 eV absorption band.

Note on the K extended X-ray absorption fine-structure Debye-Waller factor

Abstract: The cumulant expansion of the EXAFS Debye-Waller factor is discussed for the polarized and nonpolarized K EXAFS formulae. The improved formula for the nonpolarized Irs turns out to give a shorter near-neighbour distance than the value obtained by the expansion derived previously. Specifically, in CuBr at around 600 K its difference is about 0.01 AA.

Size and support effects in the photoelectron spectra of small TiO2 particles

Abstract: Three different TiO2 samples in the form of bulk material, colloidal particles and small clusters supported on SiO2 have been studied by extended x-ray adsorption fine structure (EXAFS), UV-visible, transmission electron microscopy (TEM) and XPS. The different values of the Ti 2p binding energies and the Auger parameter of Ti found for each sample show that the photoemission processes in these oxide materials are influenced by size and support effects. A difference in the relaxation energy of −1.1 eV is found for the TiO2/SiO2 sample with respect to TiO2, which is attributed to an effect of the silica support.

X-ray absorption spectroscopy in chemistry. II: X-ray absorption near edge structure

Abstract: The edges in X-ray absorption spectra exhibit fine structures designated as XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure). The information content of EXAFS with regard to interatomic distances, coordination number, and the determination of degrees of order was presented in the first part of this article [Trends Anal. Chem., 11 (1992) 218]. The second part deals with the fine structures near to the edge, the XANES region, which contains information about the electronic and the geometric structure of the absorbing atom.

Atomic configurations and local order in laser‐synthesized Si, Si‐N, Si‐C, and Si‐C‐N nanometric powders, as studied by x‐ray‐induced photoelectron spectroscopy and extended x‐ray‐absorption fine‐structure analysis

Abstract: The chemical bonding and local order in Si, Si‐N, Si‐C, and Si‐C‐N nanometric powders prepared by laser synthesis have been investigated by using two experimental methods. The local environment of Si, C, and N atoms have been studied by x‐ray‐induced photoelectron spectroscopy, from a detailed analysis of the Si 2p, C 1s, and N 1s core levels. Complementary information concerning the structural arrangement around Si atoms has been deduced from extended x‐ray‐absorption fine‐structure analysis performed at the Si K edge. The results show that the Si and Si‐C powders prepared with a 600 W laser power are pure, well‐crystallized materials; the Si‐N sample prepared with the same laser power has a disordered structure, although chemical order exists. For the composite samples, the results reveal that, in particular preparation conditions, a local C‐Si‐N3 structure is formed.

Structural Study of Tin‐Doped Indium Oxide Thin Films Using X‐Ray Absorption Spectroscopy and X‐Ray Diffraction II . Tin Environment

Abstract: Thin films of tin-doped In 2 O 3 (ITO for indium tin oxide) with different tin contents produced by a powder pyrolysis technique have been investigated by x-ray absorption spectroscopy and x-ray diffraction. In a previous paper, an extended x-ray absorption fine structure (EXAFS) study at the indium K-edge and diffraction measurements were presented. From the EXAFS investigations at the indium edge, it has been shown that the indium atomic environment becomes disordered during the doping process. In the present work, the EXAFS results at the tin K-edge and x-ray diffraction data are discussed. The EXAFS study confirms that the tin atoms are located at indium sites

Time-resolved energy-dispersive and conventional EXAFS studies of the interactions of nitrous oxide with supported copper catalyst

Abstract: The basis of the ''N2O method'' of determining the surface area of copper catalyst is quantitatively tested. At temperatures of around 60-degrees-C only a surface monolayer of Cu(I) oxide is formed: this oxide is structurally different from a monolayer slice of Cu2O. EXAFS data show that the average particle diameter of Cu on SiO2 studied here is less than 25 angstrom. On the basis of our EXAFS and other studies, the surface oxide may be pictured in terms of oxygen atoms chemisorbed at the long bridge sites of the {110} faces of metallic copper.

Electronic and atomic structure of Mn-doped CaF2: An x-ray-absorption near-edge structure and extended x-ray-absorption fine-structure study.

Abstract: X-ray-absorption near-edge structure (XANES) and extended-x-ray-absorption fine-structure (EXAFS) measurements have been performed on the calcium K edge and manganese K edge of ${\mathrm{CaF}}_{2}$ doped with manganese at molecular concentrations of 0.1--2.5 %. The results are compared to other measurements of the calcium K edge in ${\mathrm{CaF}}_{2}$ and the Mn K edge in various manganese compounds. Major features in the XANES spectra are tentatively identified with transitions to high-symmetry points of the ${\mathrm{CaF}}_{2}$ energy bands in a one-electron picture. Expected deviations from the structure predicted by the one-electron formalism are discussed. The EXAFS analysis shows a reduction in the manganese-fluorine distance of (3.9\ifmmode\pm\else\textpm\fi{}0.4)% as compared to the calcium-fluorine separation in the doped or undoped samples. This compares well with previous results obtained by EPR. A significant increase in disorder of the first shell around the Mn dopant ion is observed as compared to the Ca ion. The significance of this shell contraction and disorder is discussed in terms of possible defect structures in the material.

X-Ray absorption study on the local structures of fine particles of α-Fe2O3–SnO2 gas sensors

Abstract: X-Ray absorption spectra (EXAFS) have been recorded of the local structure around iron and tin in fine powdery α-Fe2O3–SnO2, prepared by thermal decomposition of coprecipitates from aqueous iron(III) sulfate and tin(IV) chloride solutions at pH 8 Tin atoms are substituted for iron atoms in the corundum structure of α-Fe2O3 below 15 mol% SnO2 which is the limit of dissolution Further addition of SnO2 destabilises the system such that SnO2enters the lattice as a substitutional defect At > 50 mol%, SnO2 forms a mixture of a rutile SnO2 phase and an SnO2 phase dissolved in α-Fe2O3 The Fe2O3(33 mol%)–SnO2(67 mol%) structure consists of rutile SnO2 and an irregular iron oxide phase The high sensor sensitivity for CH4 is intimately correlated with the amount of SnO2 dissolved in α-Fe2O3

Structure of hydrogenated amorphous silicon-carbon alloys as investigated by extended x-ray-absorption fine structure.

Abstract: We have studied the local structure of hydrogenated amorphous silicon-carbon alloy films, a-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{C}}_{\mathit{x}}$:H, by measuring the extended x-ray-absorption fine structure at the Si K edge. We find that first-coordination-shell average bond lengths are 2.35 A\r{} for Si-Si and 1.86 A\r{} for Si-C and are constant with concentration to within \ifmmode\pm\else\textpm\fi{}0.015 A\r{}. By comparing the composition of the first coordination shell around Si with the average concentration, we show that the alloy tends to be chemically ordered, in that heteroatomic bonds are preferred.

EXAFS analysis of vanadium oxide thin overlayers on silica prepared by chemical vapour deposition

Abstract: The structure of vanadium oxide overlayers of V2O5/SiO2 catalysts prepared by chemical vapour deposition (CVD) and impregnation methods has been determined by using vanadium K-edge EXAFS and XANES. The Fourier transform of EXAFS for the vanadium oxide overlayers obtained by CVD of VO(OC2H5)3 showed very weak peaks due to V—O—V bonds, even at the high loading level of V2O5(17 wt.%), while crystalline V2O5 gave strong peaks due to V—O and V—O—V bonds. On the basis of the EXAFS and XANES results, it has been concluded that the vanadium oxides obtained by the CVD method were present as thin overlayers on SiO2, and were stable to calcination at 773 K. On the other hand, with an impregnation method, XANES of V2O5/SiO2 at loading levels less than 5 wt.% gave peaks different from those of crystalline V2O5. It was presumed that an isolated V species was formed as reported in the literature. At loading levels >5 wt.%, XANES and EXAFS for V2O5/SiO2 catalysts prepared by the impregnation method were very similar to those of crystalline V2O5, indicating that crystallites of V2O5were formed on SiO2 in this case. This occurs because aggregates of vanadium compounds, e.g. oxalate, were formed as precursors of V2O5 during the drying process.