Showing papers on "Extended X-ray absorption fine structure published in 1987"

In Situ X-ray Absorption Study of Surface Complexes: Selenium Oxyanions on α-FeOOH

Abstract: A novel application of x-ray absorption spectroscopy has provided structural information for ions sorbed at oxide-water interfaces. As an example, in situ extended x-ray absorption fine structure (EXAFS) measurements of adsorbed selenate and selenite ions at ah α-FeOOH(goethite)—water interface have been performed; these measurements show that selenate forms a weakly bonded, outer-sphere complex and that selenite forms a strongly bonded, inner-sphere complex. The selenite ion is bonded directly to the goethite surface in a bidentate fashion with two iron atoms 3.38 angstroms from the selenium atom. Adsorbed selenate has no iron atom in the second coordination shell of selenium, which indicates retention of its hydration sphere upon sorption. This method provides direct structural information for adsorbed species at solid-liquid interfaces.

Synchrotron radiation XANES spectroscopy of Ti in minerals; effects of Ti bonding distances, Ti valence, and site geometry on absorption edge structure

Abstract: AssrRAcr The near K-edge X-ray absorption spectra of Ti in a suite of silicate and oxide minerals have been examined in an effort to improve Ti characterization in solids. The spectra consist ofelectronic excitations, occurring in the "pre-edge" region, and multiple scattering resonances on the side and top ofthe edge. The pre-edge features are insensitive to Ti-O bond length, but are sensitive to valence, occurring about 2.0 eV lower in energy in Ti3+ samples. The second pre-edge feature is also sensitive to octahedral site distortion and to the presence of tetrahedral Tia* owing to intensification created by d-p orbital mixing. The multiple scattering features shift to lower energy as Ti-O bond length increases and also change in number, intensity, and energy in response to varying ligand symmetry' Analysis ofthese spectral features suggests that tetrahedral Ti4* probably occurs only at very small concentration levels in silicates, even when the total Sia* and Al3+ concentration is well below that necessary to fill available tetrahedral sites, as in the melanite-schorlomite garnets. The levels of Ti3* are more difficult to determine because of an apparent sensitivity of the pre-edge Tio* features to bulk chemistry. However, the overall results are consistent with only small amounts of Ti3+, even in samples from localities where other analyses (wet chemical and Mossbauer) have indicated significant Ti3*.

Comparison of the structure of the manganese complex in the S1 and S2 states of the photosynthetic O2-evolving complex: an x-ray absorption spectroscopy study.

Abstract: A Mn-containing enzyme complex is involved in the oxidation of H/sub 2/O to O/sub 2/ in algae and higher plants. X-ray absorption spectroscopy is well suited for studying the structure and function of Mn in this enzyme complex. Results of X-ray K-edge and extended X-ray absorption fine structure (EXAFS) studies of Mn in the S/sub 1/ and S/sub 2/ states of the photosynthetic O/sub 2/-evolving complex in photosystem II preparations from spinach are presented in this paper. The S/sub 2/ state was prepared by illumination at 190 K or by illumination at 277 K in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU); these are protocols that limit the photosystem II reaction center to one turnover. Both methods produce an S/sub 2/ state characterized by a multiline electron paramagnetic resonance (EPR) signal. An additional protocol, illumination at 140 K, produces a state characterized by the g = 4.1 EPR signal. The authors have previously observed a shift to higher energy in the X-ray absorption K-edge energy of Mn upon advancement from the dark-adapted S/sub 1/ state to the S/sub 2/ state produced by illumination at 190 K. The Mn K-edge spectrum of the 277 K illuminated sample is similar to that produced atmore » 190 K, indicating that the S/sub 2/ state is similar when produced at 190 or 277 K. A similar edge shape and an edge shift of the same magnitude are seen for the 140 K illuminated sample. These results indicate that the g = 4.1 signal arises from oxidation of the Mn complex and that the structural differences between the species responsible for the g = 4.1 signal and the multiline EPR signal are subtle. They conclude from the edge and EXAFS studies that the light-induced S/sub 1/ to S/sub 2/ transition at 190 K or at 277 K involves a change in the oxidation state of Mn with no EXAFS-detectable change in the coordination of Mn in the O/sub 2/-evolving complex.« less

Accurate Bond Length Determination by EXAFS Method

Abstract: The Extended X-ray Absorption Fine Structure (EXAFS) spectra of pure iron, pure platinum and intermetallic compound Fe 2 Zr, whose strtuctures were well krown, have been measured at lower temperatures (20 K–300 K), and the accuracy of the interatomic-distance determination has been estimated In order to increase the accuracy in determining the values of the structural parameters, the FABM technique proposed by Teo et al has been applied In our results, however the accuracy of the interatomic distance under various temperature could be improved almost an order of magnitude by assuming that the inflection point on the absorption edge represents the edge position

Bulk and surface optical absorption in molybdenum disulfide

Abstract: Optical absorption in crystalline MoS2 has been measured from 0.7 to 3.5 eV using photothermal deflection and transmission spectroscopies. We have determined the indirect band gap in both natural and synthetic material. Photon energies below the band gap show defect absorption which is strongly concentrated at the edge surfaces. These surface defects have the same optical properties in microcrystals and lithographically textured single crystals.

Ferroelectric transition in Pb1-xGexTe: Extended x-ray-absorption fine-structure investigation of the Ge and Pb sites.

Abstract: Extended x-ray-absorption fine-structure measurements have been made on the Ge and Pb edges of ferroelectric Pb/sub 1-//sub x/Ge/sub x/Te disordered alloys. Our results show that Ge atoms are displaced approximately 0.8 A from the cell center both below and above the transition, implying that the ferroelectric transition involves long-range ordering of the dipoles. We have also measured the displacement of the Pb sublattice relative to the Te sublattice, thus showing evidence for a simultaneously order-disorder and displacive transition.

Absorption spectroscopy at the limit: detection of a single atom.

Abstract: We investigate the sensitivity limit of absorption spectroscopy. An experiment is described in which the decrease in transmitted light intensity that is due to absorption by a single, electromagnetically confined atomic ion is observed.

Quantitative in vivo absorption spectra of phytoplankton: Detrital absorption and comparison with fluorescence excitation spectra'

Abstract: Spectra1 absorption of phytoplankton from cultures and natural samples was measured on filters with various optical setups including collimated and diffuse irradiation and measurements ofwetted filters within an integrating sphere. In suspensions within an integrating sphere, specific absorption coefficients for laboratory cultures varied by a factor of only two. Measurements on filters yielded values dependent on filter load. Specific absorption coefficients derived from measurements of sample filters were considerably higher than values obtained from suspensions in an integrating sphere due to increased diffuseness of irradiance and to pathlength amplification by filter-particle and particle-particle interactions. Measured absorption of phytoplankton in the blue can be increased greatly by absorption of detritus, evident from absorption spectra of depigmented samples on filter. After subtracting detrital absorption, absorption spectra of phytoplankton are qualitatively similar to the corresponding quantum-corrected fluorescence excitation spectra. The detritus-corrected ratio of absorption at 440 vs. 675 nm shows average values between 1 and 1.5. Comparison with published values shows that specific in vivo absorption coefficients of phytoplankton arc mostly overestimated as a result of the methodology applied. In the blue region of the spectrum, overestimation of phytoplankton absorption in field samples is possible if detrital absorption is neglected.

Structural studies of the molybdenum site in the MoFe protein and its FeMo cofactor by EXAFS

Abstract: Nitrogenase is a complex bacterial enzyme system that is responsible for the conversion of atmospheric N/sub 2/ to ammonia. The structure and function of molybdenum in the MoFe protein of this system has been the subject of a number of investigations, including the use of X-ray absorption spectroscopy. This paper reports the results of the authors recent studies on several states of the MoFe protein and its FeMo cofactor (which is extruded by treatment with N-methylformamide). Mo K-edge (XANES) and extended fine structure (EXAFS) spectra have been recorded to high energies above the absorption edge with excellent signal-to-nose on the semireduced form of the MoFe protein from both Clostridium pasteurianum and Azotobacter vinelandii and on the as isolated FeMo-co and FeMo-co treated with benzenethiol and with benzeneselenol. In all of the states studied, EXAFS results reveal that the Mo is in an environment that contains two or three oxygen (or nitrogen) atoms at 2.10-2.12 A, three to five S atoms at 2.37 A, and three to four Fe atoms at 2.68-2.70 A. The numbers of these ligands change upon removal of the cofactor from the protein as discussed in the paper. For FeMo-co, comparisons also show that thil/selenol is notmore » binding directly to the Mo site. The results of these EXAFS (and their XANES published earlier) definitely show the presence of several low-Z ligands and are not compatible with a tetrahedral arrangement of only nearest S neighbors at the Mo site.« less

Extended x-ray-absorption fine structure of liquid water.

Abstract: Extended x-ray-absorption fine structure (EXAFS) of liquid water above the oxygen K edge is reported for the first time Major features of the EXAFS spectrum agree with the calculated spectrum based on the known O-O pair correlation function in water Other features are tentatively identified as due to the effect of the hydrogen atoms on the photoelectrons

X-ray absorption spectroscopic studies of silicate glasses and minerals

Abstract: Selected results of x-ray absorption spectroscopy (XAS) studies of amorphous silicates and minerals are presented in order to show their utility in providing short-and, in certain cases, medium-range structural and bonding information for cations and anions. EXAFS and XANES studies of amorphous silicates are reviewed with the objective of illustrating variations in structural environments of the various types of glass-forming cations, including Si, Al, Na, K, Ca, Ti, Fe, Yb, and U. Al is shown to occur in tetrahedral coordination in all aluminosilicate glasses examined, including peraluminous compositions. The weakly bonded Na and Ca cations are shown to occur in sites with observed coordination numbers (ranging from 6 to 7) and distances similar to those predicted by molecular dynamics simulations. Elements like Ti, which form bonds of intermediate strength, may show some order beyond the first coordination shell at low concentrations in silicate glasses. EXAFS studies of Yb and U in silicate glasses at trace to minor concentration levels provide unique structural information about the environments of these cations. K-edges and XANES of transition element sulfides, third-row tetrahedral oxyanions, and oxygen in minerals are interpreted in terms of band theory or molecular orbital theory.

X-ray absorption spectroscopic evidence for a unique nickel site in Clostridium thermoaceticum carbon monoxide dehydrogenase

Abstract: Carbon monoxide dehydrogenase (COdH) in Clostridium thermoaceticum is an ..cap alpha../sub 3/..beta../sub 3/ protein containing six nickels per molecule, in addition to a number of Fe-S clusters. Previous electron paramagnetic resonance (EPR) spectroscopic work has suggested that nickel is involved in binding CO forming an EPR-detectable species, which is probably a key intermediate in the oxidation of CO to CO/sub 2/ and in the synthesis of acetate. In order to better define the nature of this nickel site, Ni X-ray absorption spectra have been recorded and interpreted by comparison with model compounds. The extended X-ray absorption fine structure (EXAFS) spectrum for oxidized COdH is different from that of other Ni enzymes. Both the EXAFS and the X-ray absorption edge spectra suggest a Ni site containing substantial sulfur ligation. The Ni X-ray absorption edge spectrum of rubredoxin-oxidized COdH exhibits a characteristic shoulder with an inflection point at 8336 eV. Such a feature is absent in octahedral model compounds, whereas a well-resolved peak is observed in square-planar Ni complexes. A distinct 1s ..-->.. 3d transition at 8333 eV is observed in a tetrahedral model. The lack of correspondence suggests that square-pyramidal and distorted-square-planar geometries are plausible candidates for the COdH Ni site.more » Treatment of rubredoxin-oxidized COdH with hydrogenase and H/sub 2/ shifts the Ni edge to lower energies, indicating Ni-based reduction. 34 references, 3 figures.« less

Structural characterization of RNi2 (R ≡ La, Ce) intermetallic compounds and their hydrides☆

Abstract: It has been determined that the cubic phase LaNi2 (MgCu2 type) exists in a small homogeneity range (2.17 Extended X-ray absorption fine structure (EXAFS) studies at the Ni K edge for RNi2 (R ≡ La, Ce) Laves phases compounds has revealed a local disorder for both compounds which is more pronounced for LaNi2 than for CeNi2. X-ray powder diffraction measurements of the intermetallics on hydrogenation show an increase in the cell parameter for LaNi2 up to LaNi2H1 ( ΔV V = 1.4% ) and a decrease for CeNi2H1 ( ΔV V = − 0.5% ). Beyond 1 H atom mol−1 the cell parameter is almost constant. Above 3.5 H atom mol−1 the X-ray diffraction lines disappear completely. Transmission electron microscopy, magnetization and X-ray absorption spectroscopy (EXAFS and Ce LIIIedge) measurements allow us to conclude that these hydrides are amorphous (for XH⩾ 3.5). In addition, a change in the cerium valency occurs on hydrogenation.

Calcium phosphate in catheter encrustation.

Abstract: Summary— Encrusted catheters from nine female patients were the source of samples of deposits which were examined by X-ray diffraction, atomic absorption spectroscopy, infra-red spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. In eight samples the only crystalline phase which could be clearly distinguished by X-ray diffraction was ammonium magnesium orthophosphate hexahydrate, NH4MgPO4.6H2O, which occurs naturally as the mineral struvite. However, atomic absorption spectroscopy revealed an appreciable concentration of calcium in all samples. Calcium phosphates have previously been detected in catheter deposits. Infra-red and EXAFS spectra were consistent with the calcium phosphate being present as a poorly crystalline hydroxyapatite. Thus the deposits appear to consist of a mixture of crystalline struvite and a form of hydroxyapatite which is not fully crystalline.

In situ surface extended x-ray absorption fine structure spectroscopy of a lead monolayer at a silver(111) electrode/electrolyte interface

Abstract: With use of fluorescence detection and grazing incidence excitation, the X-ray absorption spectrum was obtained, at the PbL/sub III/ edge, of a monoatomic adlayer of lead on a silver (111) electrode immersed in solution. The adlayer was produced by under potential deposition from aqueous lead acetate/sodium acetate electrolyte. The edge position and the near-edge structure confirm that the lead is fully reduced to the zerovalent state. The extended X-ray absorption fine structure (EXAFS) contains no detectable contribution from lead-silver scattering, either because the lead layer is incommensurate with the underlying silver lattice or because there is large thermal motion of the lead atoms. Instead, the fine structure is due to scattering from a single type of light atom, most likely oxygen. This oxygen must arise from adsorbed water molecules or acetate ions. The lead-oxygen distance changes with the electrode potential from 2.33 +/- 0.02 A at -0.53 V to 2.38 +/- 0.02 A at -1 V (vs. Ag/AgCl, 3 M KCl).

Multiple-scattering effects in the K-edge x-ray-absorption near-edge structure of crystalline and amorphous silicon.

Abstract: 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.

Novel approach for structure analysis by x-ray Raman scattering

Abstract: An oscillation similar to that in extended x-ray absorption fine-structure (EXAFS) measurements was observed in the x-ray Raman spectra of graphite by exciting 8265-eV x rays from synchrotron radiation. From an analysis employing the formula used for EXAFS, the carbon-carbon interatomic distances are obtained which are in good agreement with the known values in graphite. Thus, x-ray Raman scattering by hard x rays is a very promising method for the determination of the local structure around light elements. For noncrystalline materials extended x-ray-absorption fine structure (EXAFS) is now an established tool for the determination of local structures around a central atom. ' An application of EXAFS to the elements with low atomic number is, however, limited to thin films or surfaces because K absorption edges lie in the soft x-ray region where the experiment is not feasible in air and adequate window materials are not available. Inelastic x-ray scattering using hard x rays may supply the same information as the absorption. It corresponds to using optical Raman scattering instead of infrared absorption. The phenomenon of x-ray Raman eff'ect has been pointed out as early as the 1920s. It was, however, only 1967 when the relationship between x-ray Raman scattering and x-ray absorption was revealed by Mizuno and Ohmura. Their theory predicts a transition probability proportional to 2

Local lattice expansion around Pd impurities in Cu and its influence on the Pd density of states: An extended x-ray-absorption fine-structure and Auger study.

Abstract: The density of states of Pd in Cu given by impurity calculations is shown to be incompatible with the observed profile of Pd Auger transitions. Extended x-ray-absorption fine-structure experiments show that there is a local expansion of the lattice around Pd impurities in Cu but not in Ag. An analysis of the influence of the local lattice expansion on the electronic structure of CuPd alloys shows that it leads to a reduction in the intensity of the Pd density of states at the bottom of the band and yields a Pd density of states in good agreement with the observed Auger profile.

Inorganic cluster compounds as models for the structure of active sites in promoted hydrodesulphurization catalysts

Abstract: Inorganic clusters of the type [Co(WS4)2NO]2–, [Co(WS4)(NO)2]–(PPh4 salts), [Co(MoS4)(NO)2]– and [Co(MoO2S2)(NO)2]–(NEt4 salts) have been synthesized and characterized by means of EXAFS, i.r. spectroscopy, X-ray diffraction and magnetic susceptibility measurements. By comparing these results with results for Co–Mo/Al2O3 hydrodesulphurization catalysts, a better understanding has been achieved of the structural and electronic environment of the active Co-edge promoter atoms (the so-called Co—Mo—S Co atoms) which can bind NO. The ‘adsorption complex’ in the catalyst has a pseudo-tetrahedral structure in which the {Co(NO)2S2} portion is diamagnetic and obeys the 18-electron rule. The promotional effect of Co is explained in terms of an increased electron density on the neighbouring S and Mo atoms, and analogies are made to a recent classification of the acid/base properties of the bulk sulphides.

Structure of bimetallic clusters. Extended x‐ray absorption fine structure (EXAFS) of Pt–Re and Pd–Re clusters

Abstract: Extended x‐ray absorption fine structure (EXAFS) studies were conducted on catalysts containing platinum and rhenium, or palladium and rhenium, on alumina. The atomic ratio of rhenium to either platinum or palladium was close to one in the catalysts investigated. The metallic entities in the catalysts were characterized by analyses of the EXAFS associated with the LI absorption edge of platinum, the LIII edge of rhenium, and the K absorption edge of palladium. It was concluded that Pt–Re and Pd–Re bimetallic clusters are present in the catalysts. However, the clusters have regions rich in rhenium and other regions which are rich in either platinum or palladium. Exposure of Pt–Re clusters to sulfur has little influence on their structure.

X-ray absorption study of the potassium coordination environment in glasses from the NaAlSi3O8−KAlSi3O8 binary: structural implications for the mixed-alkali effect

Abstract: Potassium K-EXAFS and XANES measurements were made using synchrotron radiation on selected glasses along the NaAlSi 3 O 8 -KAlSi 3 O 8 (albite-orthoclase) binary to characterize the local structural environment of potassium: Comparison of XANES spectra indicates that the potassium environment in the glasses is more similar to that in crystalline KAlSi 2 O 6 (leucite) than in crystalline KAlSi 3 O 8 (orthoclase) or KNa 3 Al 4 Si 4 O 16 (nepheline). Consequently, the potassium environment in these aluminosilicate glasses were modelled in the analysis of the EXAFS spectra using empirical phase and amplitude parameters for potassium in leucite. The derived K-O bond lengths and coordination numbers exhibit maxima near the composition albite 50 -orthoclase 50 . A model, based on previous theoretical and experimental investigations of crystalline and amorphous aluminosilicates and the mixed-alkali effect, is suggested to explain these variations in the local coordination environment of potassium across the binary alkali feldspar glass join.

X‐ray studies of concentrated aqueous solutions

Abstract: Concentrated aqueous solutions of three transition metal bromides (ZnBr2, CuBr2, and NiBr2) and an alkali bromide (RbBr) have been studied with differential anomalous scattering (DAS) and extended x‐ray absorption fine structure (EXAFS). The aq‐ZnBr2 solutions exhibit considerable inner‐shell ion complexing with the formation of tetrahedral complexes about the Zn2+. In aq‐CuBr2, the Cu2+ has an octahedral coordination shell. Most of the anions are bound directly to the cations in both solutions. In contrast, there are only a few Ni–Br nearest neighbors in aq‐NiBr2. Instead, cations and anions share hydrating water molecules. Preliminary data show that any ion complexing in aq‐RbBr must be weak. These results are in good agreement with published thermodynamic studies.