Showing papers on "Extended X-ray absorption fine structure published in 1980"
TL;DR: In this article, a procedure for determining interatomic distances, coordination numbers, and mean relative displacements from extended x-ray absorption fine structure (EXAFS) data on disordered metallic systems is described.
Abstract: A procedure is described for determining interatomic distances $r$, coordination numbers $N$, and mean relative displacements $\ensuremath{\sigma}$ from extended x-ray absorption fine structure (EXAFS) data on disordered metallic systems. This procedure includes the case when the spectrometer has finite energy resolution. It is shown how the absolute value of the EXAFS can be determined. A way has been developed to determine the inelastic loss factors of the photoelectrons from model compounds of known structure. The procedure has been checked on four metallic systems (Cu, $\ensuremath{\alpha}\ensuremath{-}\mathrm{Al}\mathrm{Cu}$-AlCu, ${\ensuremath{\theta}}^{\ensuremath{'}}$ and $\ensuremath{\theta}$ AlCu). The accuracy of the analysis is 1% for the interatomic distances, 15% for the coordination numbers, and 20% for $\ensuremath{\sigma}$. The reliability of the data analysis, which depends strongly on the number of different atoms in a shell, on the magnitude of their mean relative displacement and on the difference in their backscattering power will be discussed at length.
285 citations
TL;DR: The X-ray Absorption Near Edge Structures (XANES) of Mn compounds at the Mn K-edge have been measured with high resolution at the Frascati Synchrotron Radiation Facility "PULS" as discussed by the authors.
228 citations
166 citations
TL;DR: In this paper, a modified version of the EXAFS equation with many-body effects was proposed to obtain more accurate structure determination when standards used for comparison do not closely approximate the unknown.
Abstract: Careful extended x-ray absorption fine structure (EXAFS) measurements at various temperatures have been made on Pt, Cu, Mn${\mathrm{Cl}}_{2}$, Fe${\mathrm{Cl}}_{2}$, Co${\mathrm{Cl}}_{2}$, Ge, GaAs, ZnSe, and CuBr. Care was taken to minimize experimental errors in the measurements. The thermal variation in the disorder about the average shell distance was determined and found to agree well with calculation. Analysis of EXAFS data showed explicitly that the standard independent particle model formulation of the EXAFS equation is in error. The measurement can be accommodated by modifying the equation and adding many-body effects. The modifications consisted of adding the overlap factor ${S}_{0}^{2}$ which introduces a dependence on the type of center atom and subtracting from the path length of the photoelectron a term $\ensuremath{\Delta}$ in the exponential containing the mean free path $\ensuremath{\lambda}$. It is found that the atomic value of ${S}_{0}^{2}$ enters into EXAFS, that $\ensuremath{\lambda}$ varies significantly from material to material, and that $\ensuremath{\Delta}\ensuremath{\approx}{r}_{1}$, the distance to the first coordination shell of atoms. The new EXAFS equation containing many-body effects permits more accurate EXAFS structure determination when standards used for comparison do not closely approximate the unknown.
153 citations
TL;DR: In this paper, the physical information that can be extracted from the surface X-ray absorption near edge structures (XANES) and surface extended Xray absorption fine structures (EXAFS) is discussed.
150 citations
TL;DR: In this article, a study was made of the extended x-ray absorption fine structure (EXAFS) associated with the K absorption edges of ruthenium and copper in a silica supported RUThenium-copper catalyst.
Abstract: A study was made of the extended x‐ray absorption fine structure (EXAFS) associated with the K absorption edges of ruthenium and copper in a silica supported ruthenium–copper catalyst containing 1.0 wt.% ruthenium and 0.63 wt.% copper (1:1 atomic ratio of ruthenium to copper). Previous chemisorption and catalysis studies indicated a strong interaction between the ruthenium and copper in the catalyst, and led to the suggestion that the catalyst consisted of bimetallic clusters of ruthenium and copper dispersed on the silica. The EXAFS data of this investigation provide information on the structure of the ruthenium–copper clusters. The ruthenium in the bimetallic clusters appears to be very similar to the ruthenium in a silica supported ruthenium reference catalyst, in that the ruthenium atoms are coordinated predominantly to other ruthenium atoms and only to a minor extent to copper atoms. By contrast, the copper atoms in the clusters have nearest neighbors which are more nearly equally distributed among c...
141 citations
TL;DR: In this article, the authors examined the coordination geometry of the Ti4+ ion and concluded that this geometry is primarily a four-fold coordination with a small fraction of the ions having sixfold coordination and a longer TiO2 bond length.
Abstract: Evidence from both the extended X-ray absorption fine structure (EXAFS) and near-edge regions of the Ti K-edge X-ray absorption spectrum for TiO2SiO2 glasses has been examined to determine the coordination geometry of the Ti4+ ion. It is concluded that this geometry is primarily a four-fold coordination, with a small fraction of the Ti4+ ions having six-fold coordination and a longer TiO bond length. The ratio of six-fold to four-fold coordination is seen to increase with increased TiO2 content in the glass.
111 citations
Journal Article•
TL;DR: In this article, the manganese associated with photosynthetic oxygen evolution is suggested to occur as a bridged transition metal dimer with most likely another manganizer, and extensive details on the analysis are included.
Abstract: Extended X-ray Absorption Fine Structure (EXAFS) studies on the manganese contained in spinach chloroplasts and on certain di-u-oxo bridged manganese dimers of the form (X{sub 2}Mn)O{sub 2}(MnX{sub 2} (X=2,2'-bypyridine and 1,10-phenanthroline) are reported. From these studies, the manganese associated with photosynthetic oxygen evolution is suggested to occur as a bridged transition metal dimer with most likely another manganese. Extensive details on the analysis are included.
106 citations
TL;DR: In this paper, an extended x-ray absorption fine structure measurement was performed for aqueous solutions of CuBr2, ZnBr2 and NiCl2 at various concentrations.
Abstract: Extended x‐ray absorption fine structure measurements are reported for aqueous solutions of CuBr2, ZnBr2, CuCl2, NiCl2, and NiBr2 at various concentrations. It is shown that, for highly concentrated solutions of CuBr2, ZnBr2, and CuCl2, some local order, resembling that of the corresponding crystals, exists. When the concentration decreases, this order partly remains, while the hydration increases. In contrast to this behavior, SrBr2 does not show the existence of any bonding, and it is difficult to make conclusions in the case of NiCl2 and NiBr2 solutions. The temperature dependence of the EXAFS spectra of CuBr2 and ZnBr2 is presented also; some change in the low energy region occurs between 45 and 60°C for CuBr2 solutions.
103 citations
TL;DR: In this article, a procedure to extract the optical-absorption coefficient as a function of energy from measurements of photoconductivity and assess the assumptions involved in applying it to $a$-Si: H is presented.
Abstract: We outline a procedure to extract the optical-absorption coefficient as a function of energy from measurements of photoconductivity and assess the assumptions involved in applying it to $a$-Si: H. Using this procedure, we obtain accurate absorption spectra in the range from 1.8 to below 1.2 eV. These spectra show an absorption shoulder around 1.3 eV which we analyze in terms of possible optical transitions involving specific distributions of states in the gap.
95 citations
TL;DR: In this article, the local structure of iron monomers and dimers isolated in a rare gas Ar matrix using extended X-ray absorption fine structure measurements was investigated and compared with Mossbauer measurements on such systems.
TL;DR: In this paper, an anharmonic oscillator model is applied to the EXAFS data of polycrystalline Zn to correct the values of R1 and σ12.
Abstract: Extended X-ray absorption fine structure (EXAFS) results are presented for solid and liquid Zn for the temperature range 273 to 782 K. At the higher temperatures the distribution of nearest neighbour atoms is asymmetric. Neglect of the asymmetry and use of EXAFS analysis methods developed for symmetric distribution functions gives an underestimate of the nearest neighbour distance R1 and the disorder parameter σ12. It is shown that a transform analysis can be useful in detecting asymmetric effects, and methods of analyzing the EXAFS of asymmetric distributions are presented. An anharmonic oscillator model is applied to the EXAFS data of polycrystalline Zn to correct the values of R1 and σ12. The corrected values of σ12 are found to be~20% higher than those predicted by a Debye model. An empirical, analytically simple asymmetric distribution function is fitted to the liquid data to obtain promising agreement with the nearest neighbour structure as deduced from X-ray diffraction studies. Finally it is shown...
TL;DR: In this article, the authors investigated the oxidation of Al(111) surfaces by means of polarization-dependent surface extended-x-ray-absorption-fine-structure (EXAFS) measurements.
Abstract: We have investigated the oxidation of Al(111) surfaces by means of polarization-dependent surface extended-x-ray-absorption-fine-structure (EXAFS) measurements. The experiments were performed above the oxygen $K$ absorption edge (\ensuremath{\sim}535 eV) using the total electron-yield detection technique. The oxidation range from monolayer chemisorption corresponding to an ordered oxygen (1 \ifmmode\times\else\texttimes\fi{} 1) overlayer to the formation of a thick surface layer of amorphous oxide has been studied. For the chemisorbed monolayer (100-150-L ${\mathrm{O}}_{2}$) we find a strong polarization dependence of the EXAFS signal which allows the determination of the O-O intraoverlayer separation 2.90 \ifmmode\pm\else\textpm\fi{} 0.05 \AA{}. The O-Al chemisorption bond length has been determined to be 1.79 \ifmmode\pm\else\textpm\fi{} 0.05 \AA{}. Within experimental error we find the same O-Al distance for the initial oxide-like phase which accompanies the chemisorbed phase. At higher exposures (1000-L ${\mathrm{O}}_{2}$) the vanishing low-energy-electron-diffraction pattern is accompanied by an increase in the O-Al bond length (1.88 \ifmmode\pm\else\textpm\fi{} 0.03 \AA{}). This value is also observed for heavily oxidized surfaces. The O $K$ edge shows the formation of a strong "white-line"-like threshold resonance with increasing oxygen exposure. This resonance is attributed to transitions to localized final states in ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$-like molecular units. It is pointed out that detailed analysis of the absorption threshold may by itself provide information on the local structure of surface complexes. Models for the progressive oxidation process of Al(111) surfaces implied by our data are discussed.
TL;DR: The absorption spectra of CdIn2S4 have been measured at various temperatures between 47 K and 412 K, and analyzed in the region of the absorption coefficient from 1 to 2?104 cm-1.
Abstract: The absorption spectra of CdIn2S4 have been measured at various temperatures between 47 K and 412 K, and analyzed in the region of the absorption coefficient from 1 to 2?104 cm-1 The data are interpreted as indicating there is direct-allowed transition with an energy gap of 275 eV at 0 K Indirect-allowed transition is also observed with an energy gap of 244 eV at 0 K The temperature-modulated optical absorption measurement of CdIn2S4 indicates that this material has an indirect energy gap of 228 eV at room temperature and an assisting phonon of 003 eV
TL;DR: In this paper, a method for determining the absolute absorption coefficient of condensed samples is presented. But this method is not applicable to the case of ion electrophoresis in aqueous solutions.
Abstract: In order to perform quantitative photoacoustic spectroscopy (PAS) it is desirable to have a method of obtaining the absolute absorption coefficient of a sample from PAS measurements alone. The photoacoustic signal contains both amplitude and phase data and this paper discusses a method by which this information may be used to determine the absolute absorption coefficient of condensed samples. Experimental results on ions in aqueous solution are also presented.
TL;DR: In this article, absorption and extended-fine-structure studies on the carbon $K$ edge in micro-crystalline, poly-crystaline, and oriented crystalline (monochromator) graphite were performed.
Abstract: We report absorption and extended-fine-structure studies on the carbon $K$ edge in microcrystalline, polycrystalline, and oriented crystalline (monochromator) graphite. On the basis of a density-of-states analysis, strong excitonic effects about the core hole are inferred. The fine structure at the absorption edge is found to differ for the three materials. This is explained by the orientational dependence of the polarization selection rules for electric dipole transitions from an $s$ core to $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ conduction-band states. The extended x-ray fine structure (EXAFS) above the edge exhibits similar oscillations for the bulk crystalline samples (polycrystalline and monochromator graphite), but considerably weaker structure is observed for the microcrystalline sample. A Fourier analysis of the oscillations yields the nearest-neighbor shell separations. Finally, it is shown that the intensity modulations caused by carbon contamination on optical Au and Pt mirror surfaces that are exposed to high-intensity synchrotron radiation closely resemble those above the C $K$ edge in bulk crystalline graphite. This indicates the formation of graphitic overlayers on such mirrors even under ultrahigh-vacuum (1 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}9}$ Torr) conditions. These findings furthermore demonstrate the feasibility of EXAFS studies of adsorbates on surfaces by monitoring the (totally) reflected radiation off the substrate.
TL;DR: In this article, the x-ray absorption spectra of Ge and of Br in NaBr have been measured to pressures of 52 and 21 kbars, respectively, in a boron carbide and diamond anvil cell in which pressure was measured via the ruby‐fluorescence technique.
Abstract: The x‐ray absorption spectra of Ge and of Br in NaBr have been measured to pressures of 52 and 21 kbars, respectively, in a boron carbide and diamond anvil cell in which pressure was measured via the ruby‐fluorescence technique. Although Bragg peaks from the diamond anvil reduced the accuracy, atomic spacings in both materials could be determined by extended x‐ray absorption fine‐structure (EXAFS) analysis. Changes in the nearest‐neighbor separations in NaBr, and Ge to at least 40 kbars, agreed with literature values, indicating that the EXAFS phase shifts are quite insensitive to such pressures. In addition the near‐edge peak positions in the NaBr spectra appeared to readily shift with pressure, which suggests that NaBr may be quite suitable as a pressure standard in future work of this type.
TL;DR: Experimental procedures of soft x-ray absorption and extended absorption fine structure (EXAFS) measurements as well as applications and future prospects of such studies are emphasized.
Abstract: Absorption measurements above the K edges of the low-Z atoms, C, N, O, and F, are discussed, which utilize 250–1000-eV synchrotron radiation. The paper emphasizes experimental procedures of soft x-ray absorption and extended absorption fine structure (EXAFS) measurements as well as applications and future prospects of such studies. In particular, we discuss such applications as surface EXAFS, surface absorption fine structure, and ion yield EXAFS measurements.
TL;DR: In this article, Fourier filtering and fitting the normalized glass spectra using α-ZnCl2 as a model compound was determined to be (2.34 ± 0.01) A and the average coordination number about the Zn2+ was found to be 5.1 ± 0.8.
Abstract: The Zn-EXAFS (extended X-ray absorption fine structure) above its K-absorption edge has been measured for glassy ZnCl2 at low temperature, through Tg (375 K), and into the supercooled and normal liquid state (mp = 593 K) at 673 K. By Fourier filtering and fitting the normalized glass spectra using α-ZnCl2 as a model compound, the Zn2+Cl− distance was determined to be (2.34 ± 0.01) A and the average coordination number about the Zn2+ was found to be 5.1 ± 0.8. The latter value agrees with the value of 4.7 nearest neighbors obtained by the molecular dynamics computer simulation study of Woodcock et al., for liquid ZnCl2 just above its melting point. The agreement is supportive of the notion that short-range order in the glass is reflective of that of the corresponding liquid from which it was quenched. Spectral measurement as a function of temperature indicates that there is considerable dynamic decoupling of the Zn2+ from its nearest Cl− neighbor even below Tg. This is contrasted with similar data in glassy GeO2 which show that the motion of Ge is strongly coupled with its four oxygen neighbors all the way to Tg. This difference in dynamic disorder substantiates the notion that glassy ZnCl2, as well as vitreous BeF2, provides a further weakened structural analog for glassy GeO2 and SiO2.
TL;DR: In this article, the x-ray absorption spectrum of the Sm L/sub III/ edge has been measured in mixed-valence Sm/sub 0.75/Y/Sub 0.25/S. From analysis of the edge structure, the valence is determined and it is shown that the S neighbors of each Sm atom adopt an average distance rather than a dynamically distorted environment with two distances corresponding to the two valence states.
Abstract: X-ray absorption spectrum of the Sm L/sub III/ edge has been measured in mixed-valence Sm/sub 0.75/Y/sub 0.25/S. From analysis of the edge structure and extended x-ray absorption fine structure, the valence is determined and it is shown that the S neighbors of each Sm atom adopt an average distance rather than a dynamically distorted environment with two distances corresponding to the two valence states. From this it is concluded that the characteristic Sm 4f-band width is not greatly modified by polaron effects.
TL;DR: In this article, a bimetallic Ru-Au catalysts supported on MgO were prepared, their atomic composition varying from 100% Ru to 100% Au, and they were characterized after impregnation and drying at 110 °C and after further reduction by hydrogen at 400 °C.
TL;DR: In this article, two platinum catalysts (6.3% Pt on SiO2, dispersion > 50, designated EUROPT-1 and 0.47% PT on Al2O3, designated PTAL) have been studied by extended X-ray absorption fine structure (EXAFS).
Abstract: Two platinum catalysts (6.3% Pt on SiO2, dispersion > 50%, designated EUROPT-1 and 0.47% Pt on Al2O3, designated PTAL) have been studied by extended X-ray absorption fine structure (EXAFS). Each Fourier transform is dominated by a single radial distance, assigned to Pt—O bonding. In EUROPT-1 the Pt—O distance is 1.94 ± 0.05 A, slightly shorter than the Pt—O distance in βPtO2, 2.00 ± 0.02 A. For PTAL a broad peak is observed centred at 2.00 ± 0.05 A, which may contain a contribution due to Pt—C1 bonding. No evidence is found of metallic platinum or the Pt—Pt bonds in βPtO2. The platinum particles are thus largely amorphous, consisting of PtOx units (x⩽ 6). The comparitive ease of oxidation of small highly disperse platinum particles is discussed.
TL;DR: In this paper, the fine structure on the LIIIX-ray absorption edge of Au using calculated phase shifts was analyzed and compared with that of Na3Au(S2O3)2, a compound of known structure.
Abstract: Analysis of the fine structure on the LIIIX-ray absorption edge of Au using calculated phase shifts shows that in disodium thiomalato-S-gold(I)(‘Myocrisin’) and thioglucopyranosyl-S-gold(I)(‘Solganol’) the gold(I) atom is bonded to two (bridging) S atoms 2·37 A; away; EXAFS spectra are compared with that of Na3Au(S2O3)2, a compound of known structure.
TL;DR: In this article, site selection experiments have been performed on pheophytin-a and chlorophyll-a in n-octane at 4.2 K. The absorption spectra show sharp lines and vibrational frequencies of the lowest excited singlet state correspond with resonance Raman frequencies.
TL;DR: The first opto-acoustic absorption spectra of liquid methane at 94 K in the region of 5,500-7,500 A was reported in this paper, with peak absorption coefficients of 16 × 10−3 to 1 × 10 −4 cm−1.
Abstract: The first opto-acoustic absorption spectra of liquid methane at 94 K in the region of 5,500–7,500 A is reported here. The observed absorption bands are very weak with peak absorption coefficients of 16 × 10−3 to 1 × 10−4 cm−1 and are of importance in the study of spectra of outer planets (such as Jupiter and Neptune) for detecting the existence of liquid methane on these planets.
TL;DR: In this paper, a broadband continuous-wave dye laser has been enclosed to eliminate interfering atmospheric absorption, and has been run at pressures as low as 50 Torr of various gases.