Showing papers on "Extended X-ray absorption fine structure published in 1995"

Multiple-scattering calculations of x-ray-absorption spectra

Abstract: A high-order multiple-scattering (MS) approach to the calculation of polarized x-ray-absorption spectra, which includes both x-ray-absorption fine structure and x-ray-absorption near-edge structure, is presented. Efficient calculations in arbitrary systems are carried out by using a curved-wave MS path formalism that ignores negligible paths, and has an energy-dependent self-energy and MS Debye-Waller factors. Embedded-atom background absorption calculations on an absolute energy scale are included. The theory is illustrated for metallic Cu, Cd, and Pt. For these cases the MS expansion is found to converge to within typical experimental accuracy, both to experiment and to full MS theories (e.g., band structure), by using only a few dozen important paths, which are primarily single-scattering, focusing, linear, and triangular.

Two-photon absorption and optical-limiting properties of novel organic compounds

Abstract: The optical-limiting behavior and two-photon absorption properties of four novel organic compound solutions in tetrahydrofuran have been investigated. An ultrashort laser source with 0.5-ps pulse width and 602-nm wavelength was employed. The transmissivities of the various 1-cm-thick solution samples have been measured as a function of the beam intensity as well as of the solute concentration. The measured results can be fitted on the assumption that two-photon absorption is the only predominant mechanism causing the observed opticallimiting behavior. Based on the intensity-dependent transmissivity measurements, the molecular two-photon absorption coefficients for the four compounds are presented.

Cavity ring-down spectroscopy for quantitative absorption measurements

Abstract: We examine under what conditions cavity ring-down spectroscopy ~CRDS! can be used for quantitative diagnostics of molecular species. We show that CRDS is appropriate for diagnostics of species whose absorption features are wider than the spacing between longitudinal modes of the optical cavity. For these species, the absorption coefficient can be measured by CRDS without a knowledge of the pulse characteristics provided that the cavity ring-down decay is exponential. We find that the exponential ring-down decay is obeyed when the linewidth of the absorption feature is much broader than the linewidth of the light circulating in the cavity. This requirement for exponential decay may be relaxed when the sample absorption constitutes only a small fraction of the cavity loss and, consequently, the sample absorbance is less than unity during the decay time. Under this condition the integrated area of a CRDS spectral line approximates well the integrated absolute absorption coefficient, which allows CRDS to determine absolute number densities ~concentrations!. We determine conditions useful for CRDS diagnostics by analyzing how the absorption loss varies with the sample absorbance for various ratios of the laser pulse linewidth to the absorption linewidth for either a Gaussian or a Lorentzian absorption line shape. © 1995 American Institute of Physics.

X-ray-absorption spectroscopy and n-body distribution functions in condensed matter. II. Data analysis and applications.

Abstract: The practical and theoretical aspects of the GNXAS method for multiple-scattering extended x-ray-absorption fine-structure (EXAFS) data analysis are treated in a comprehensive account. The model function used to fit the raw absorption coefficient is described and details on the least-squares fitting procedure and parameter definition are reported. Large emphasis is given, in EXAFS analysis, to the description of criteria for a complete statistical evaluation of the results, including error estimate and model evaluation. An extensive set of applications to prototypical molecular (${\mathrm{Br}}_{2}$, ${\mathrm{CS}}_{2}$), and crystalline [c-Ge and Pd (fcc)] systems is reported. Structural parameters always coincide with the known values within statistical accuracy indicating that systematic errors due to approximations in the theory are negligible. The present results also demonstrate that x-ray absorption spectroscopy can provide information on three-body atomic arrangements like average geometrical and vibrational parameters with statistical significance. In all examples, despite the inclusion of several three-body contributions, the total number of fitting parameters does not exceed the information content of the spectra; details on error evaluation and correlation plots in the parameter space are reported.

Solid State EXAFS and Luminescence Studies of Neutral, Dinuclear Gold(I) Complexes. Gold(I)-Gold(I) Interactions in the Solid State

Abstract: A series of neutral, dinuclear gold(I) complexes, containing phosphine and thiolate ligands, have been studied by EXAFS and luminescence spectroscopy. Gold(I)-gold(I) interactions are detected for the first time by EXAFS studies on solid samples at liquid helium temperature. A strong, distinct peak at 1.90 {+-} 0.02 {angstrom}, assigned to Au-P and Au-S bonds, appears in the Fourier transform for complexes 1-8. A less intense peak appears for complexes 2-6 at 2.8 {angstrom} with the amplitude maximizing toward high k characteristic of a gold backscattering atom. The calculated EXAFS results indicate gold(I)-gold(I) distances ranging from 3.0 to 3.2 {angstrom} for complexes 2-6. In contrast, no gold(I)-gold(I) interactions are detected for complexes 1, 7, and 8. The Au-Au and Au-P(S) distances calculated by EXAFS are similar to those measured by X-ray diffraction. All of the neutral, dinuclear gold(I) complexes luminescence at room temperature in the solid state. The Stokes shifts average 6 x 10{sup 3} cm{sup {minus}1} and are indicative of a large distortion in the excited state compared to the ground state. Spectral acquisition using time delays of 10-50 {mu}s confirms the phosphorescent nature of the emission. The origin of the luminescence of complexes 1-8 is consistent with a S {yields}more » Au CT excited state that is perturbed by substituent electronic effects leading to the red shift in emission for 5-8 relative to 1-4. There is no correlation between gold(I)-gold(I) bonding and the energy or band shape of the excitation and emission of 1-8. The luminescence and EXAFS results taken together demonstrate that a gold(I)-gold(I) interaction is not a necessary condition for luminescence. Further, the presence of a gold(I)-gold(I) interaction does not significantly perturb the luminescence in this series of gold(I) complexes.« less

Blue photoluminescence and local structure of Si nanostructures embedded in SiO2 matrices

Abstract: Strong and stable blue photoluminescence (PL), visible to the naked eye under 0.4 μW of 300 nm and 2.7 μW of 370 nm excitation, has been observed for samples of Si clusters embedded in SiO2 matrices, prepared by rf co‐sputtering followed by N2 annealing at 800 °C. Si K‐edge extended x‐ray absorption fine structure (EXAFS) and near‐edge x‐ray absorption fine structure (NEXAFS) strongly suggest the existence of Si nanoclusters with crystalline cores in the efficient emitting material. The PL excitation dependence is explained by an increase in the conduction band density of states deep in the band, and the formation of a band tail.

Ultralow values of the absorption coefficient of Si obtained from luminescence.

Abstract: Photoluminescence spectra were measured on homogeneously doped silicon at temperatures of 90 and 295 K. By using the generalized Planck law for the emission of luminescence by indirect transitions, values for the absorption coefficient as small as ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ have been obtained for photon energies well below the band gap. At higher photon energies, where absorption coefficients can be obtained from transmission, good agreement with literature data is observed.

Structural consequences of ammonia binding to the manganese center of the photosynthetic oxygen-evolving complex: an X-ray absorption spectroscopy study of isotropic and oriented photosystem II particles.

Abstract: The structure and orientation of the manganese complex in NH3-treated photosystem II (PS II) membrane particles of spinach are being studied by X-ray absorption spectroscopy. On the basis of earlier work by our group, a structure for the tetranuclear manganese complex of PS II, which consists of two di-mu-oxo-bridged binuclear Mn units linked by a mono-mu-oxo group, has been proposed [Yachandra, V. K., et al. (1993) Science 260, 675-679]. The extended X-ray absorption fine structure (EXAFS) of the complex modified by NH3 binding in the S2-state is suggestive of an increase in the Mn-Mn distance of one of these units from 2.72 +/- 0.02 to 2.87 +/- 0.02 A, whereas the Mn-Mn distance of the second unit seems to be unaffected by NH3 treatment. The elongation of one binuclear center could result from the replacement of one bridging mu-oxo by an amido group. The lengthening of one Mn-Mn distance means that, by NH3 treatment, the distance degeneracy of the 2.7 A Mn-Mn EXAFS interaction is removed. Consequently, the orientation of individual binuclear units with respect to the membrane normal becomes resolvable by EXAFS spectroscopy of partially oriented PS II membrane particles. The angle between the normal of the PS II-containing membrane and the Mn-Mn vector is determined to be 67 degrees +/- 3 degrees for the 2.87 A distance and 55 degrees +/- 4 degrees for the 2.72 A distance. Only small effects on position, shape, and orientation dependence of Mn K-edge spectra result from NH3 treatment, indicating that the Mn oxidation state, the symmetry of the Mn ligand environment, and the orientation of the complex remain essentially unaffected in the annealed NH3 S2-state. Therefore, it seems likely that the angles determined for the ammonia-modified manganese complex are similar to the respective angles of the untreated complex. The structure of the manganese complex and its orientation in the membrane are discussed.

Absorption edge of silicon from solar cell spectral response measurements

Abstract: The optical absorption coefficient of crystalline silicon near the band edge is determined to values as low as 10−7 cm−1 by sensitive photocurrent measurements on high efficiency silicon solar cells. Structure due to three‐ and four‐phonon assisted absorption processes is observed. Discrepancies between absorption coefficient values around 10−2 cm−1 reported in the literature are resolved. The role of disorder theory in understanding the absorption edge of crystalline semiconductors such as silicon is discussed.

Evidence for the proximity of calcium to the manganese cluster of photosystem II: determination by X-ray absorption spectroscopy.

Abstract: The photosynthetic oxygen-evolving complex contains a cluster of four manganese atoms and requires both Ca and Cl for activity. The question of Ca proximity to the Mn cluster has been investigated by performing Mn X-ray absorption experiments on native samples of photosystem II (PS II) and on samples depleted of Ca and reconstituted by either Ca or Sr. Analysis of X-ray K-edge spectra demonstrates no significant differences in oxidation state or symmetry between Ca- and Sr-reactivated preparations. Differences are observed in the extended X-ray absorption fine structure (EXAFS). The amplitude of a Fourier transform peak due to scatterers at distances greater than 3 {angstrom} is larger for samples reactivated with strontium than for calcium-reactivated samples. Taking into account the stoichiometry of Mn and Ca atoms in PS II and considering physically reasonable structures, curve-fitting analyses of the EXAFS data using FEFF5-calculated parameters favor a model where both manganese and calcium (or strontium) scatterers contribute to the Fourier peak at approximately 3 {angstrom}. Other models for the {approximately}3 {angstrom} peak with multiple Mn-Mn interactions or multiple Mn-Ca(Sr) interactions can also be fit to the data, but are considered less likely. This result provides confirmation for the structural proximity of Ca tomore » the Mn cluster suggested previously. Possible structural arrangements for a calcium-binding site are discussed. 53 refs., 7 figs., 5 tabs.« less

Multielectron excitations at the L edges in rare-earth ionic aqueous solutions

Abstract: An extensive investigation of the extended x-ray-absorption fine structure (EXAFS) at the L edges of the rare-earth atoms of aqueous ionic solutions of ${\mathrm{La}}^{3+}$, ${\mathrm{Ce}}^{3+}$, ${\mathrm{Ce}}^{4+}$, ${\mathrm{Nd}}^{3+}$, ${\mathrm{Pr}}^{3+}$, ${\mathrm{Eu}}^{3+}$, ${\mathrm{Dy}}^{3+}$, and ${\mathrm{Tm}}^{3+}$ at concentrations of 50, 100, and 200 mM, is presented. The presence of anomalous peaks, appearing in the range from 5 to 7 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$ and superimposed to the main single-frequency oscillatory signal, has been explained as due to double-electron transitions 2p4d\ensuremath{\rightarrow}5${\mathit{d}}^{2}$ in the case of ${\mathit{L}}_{3}$ and ${\mathit{L}}_{2}$ edges, and 2s4d\ensuremath{\rightarrow}6p5d for the ${\mathit{L}}_{1}$ spectra. The energy of the double-excitation absorption edge increases as the atomic number of the element of the rare-earth series is increased and is in fair agreement with previous theoretical bound-to-bound calculations. The intensity of the anomalous feature decreases for increasing Z numbers, as expected from theory, but the intensity values, calculated from comparison with the main single-electron absorption line, are lower than those calculated by other authors and the double-excitation peak disappears in the ${\mathrm{Tm}}^{3+}$ spectrum. A structural analysis of the EXAFS spectra was also carried out with the twofold aim of characterizing rare-earth water solutions and quantifying the errors introduced in the structural parameters by the mixing of single- and double-electron phenomena. The results show the rare-earth ions are always surrounded by 12 water molecules and the rare-earth--O distance decreases with Z number, varying from 2.56 \AA{} for ${\mathrm{La}}^{3+}$ down to 2.32 \AA{} for ${\mathrm{Tm}}^{3+}$. The presence of the anomalous peaks introduces small errors in the bond-length's determination, the effect being proportional to the magnitude of the double-excitation peak.

Metal incorporation in sputter-deposited MoS2 films studied by extended x-ray absorption fine structure

Abstract: Solid lubricant films produced by cosputtering metals with MoS2 and by forming metal/MoS2 multilayers are being planned for use in the next generation of solid lubricated devices on spacecraft, including gimbal and sensor bearings, actuators, and sliding electrical contacts The films exhibit increased densities and wear lives compared to films without additives, but the mechanism of density enhancement is not well understood The extended x-ray absorption fine structure (EXAFS) technique is ideal for elucidating the structure of these poorly crystalline films We analyzed MoS2 films cosputtered with 0, 2, and 10% Ni, as well as Ni/MoS2 and Au(Pd)/MoS2 multilayer films The results obtained at the Mo-K absorption edge showed that the metal-containing films comprised predominantly the same nanocrystalline phases present in similar films without added metals: pure MoS2 and a MoS2−xOx phase MoS2−xOx is isostructural with MoS2, with O atoms substituting for S atoms in the MoS2 crystal lattice For all Ni-containing films, EXAFS data obtained at the Ni-K absorption edge showed that the Ni had not chemically reacted with the MoS2−xOx and MoS2, but formed a disordered NiOx phase However, Ni-cosputtered films showed decreasing Mo-Mo bond lengths in the MoS2−xOx phase with increasing Ni content, probably due to preferential oxidation of Ni compared to MoS2 EXAFS of these Ni-cosputtcred films showed only a small decrease in short-range order with Ni content, while x-ray diffraction showed a concurrent large decrease in long-range order The results indicate that film densification in Ni-cosputtered films is caused by NiOx formation at the edges of nucleating MoS2−xOx/MoS2 crystallites, limiting the crystallite size attainable within the films

Low Z elements (Mg, Al, and Si) K-edge X-ray absorption spectroscopy in minerals and disordered systems

Abstract: Soft X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy have been performed at the Mg-, Al- and Si-K edges in order to establish the ability of this spectroscopy to derive structural information in disordered solids such as glasses and gels. Mg- and Al-K XANES are good structural probes to determine the coordination state of these elements in important minerals, glasses and gels. In a CaO MgO 2SiO2 glass Mg XANES spectra differ from that found in the crystalline equivalent, with a significant shift of the edge maxima to lower energy, consistent with a CN lower than 6. Mg-EXAFS on the same sample are in agreement and indicate the presence of 5-coordinated Mg with Mg O distances of 2.01A. In aluminosilicate gels, Al K XANES has been used to investigate the [4]Al/Altotal ratios. These ratios increase as the Al/Si ratios decrease. Aluminosilicate and ferric-silicate gels were studied by using Si K edge XANES. XANES spectra differ significantly among the samples studied. Aluminosilicate gels with Al/Si= 1 present a different Al and Si local environment from that known in clay minerals with the same Al/Si ratio. The gel-to-mineral transformation thus implies a dissolution-recrystallization mechanism. On the contrary, ferric-silicate gel presents a Si local environment close to that found in nontronite which may be formed by a long range ordering of the initial gels.

Coprecipitated NiAl and NiCuAl catalysts for methane decomposition and carbon deposition I. Genesis of Calcined and reduced catalysts

Abstract: The structure of coprecipitated NiAl and NiCuAl catalyst with high nickel content (60–90 wt.-% Ni-alumina) has been investigated by X-ray powder diffraction, extended X-ray absorption fine structure (EXAFS) spectroscopy and Auger electron spectroscopy of calcined and reduced samples. The alumina was shown to behave as a textural promoter. The model of the reduced NiAl catalyst consisted of highly dispersed nickel and alumina particles with a NiAl spinel phase at their interface. The NiAl spinel phase had mainly an inverse structure. In the reduced sample the paracrystallinity of nickel was evidenced by precise diffraction measurements using synchrotron radiation. The spinel structure was not observed when copper was introduced. As a result, the alumina particles were more dispersed and covered the surface of the NiCu particles enriched with copper.

A Si K-edge EXAFS/XANES study of sodium silicate glasses

Abstract: Si K-edge X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) have been obtained on a series of sodium silicate glasses containing 15–40 mol% Na2O. The XANES reveals that, with the addition of Na2O, the SiSi bond distance distribution decreases and there are increased contributions from multiple scattering beyond the second coordination sphere. These responses imply that some degree of network ordering occurs with alkali addition. The EXAFS data indicate that the SiO bond distance increases from 1.61 ± 0.02 A for amorphous SiO2 to 1.66 ± 0.02 A for 30 mol% added Na2O. For Na2O > 30 mol%, the SiO bond distance decreases. The SiO bond distance changes indicate that, for ≤ 30 mol% Na2O, network depolymerisation effects on the SiO bond dominate any effect from increased non-bridging oxygen (NBO) formation. For compositions > 30 mol% Na2O, increased NBO formation has the dominant effect on the SiO bond distance. This may indicate that the microsegregation of network modifiers from network formers, as predicted by molecular dynamics studies, is significant at ≥ 30 mol% added Na2O.

Characterization and orientation of adsorbed NO dimers on Ag{111} at low temperatures

Abstract: Reflection‐absorption infrared spectroscopy (RAIRS) and near‐edge x‐ray absorption fine structure(NEXAFS) have been used, with isotopic 14NO/15NO mixtures, to determine the structure and orientation of the monolayer species formed by NO adsorption on Ag{111} at 40 to 60 K. The adlayer is composed of NO dimers bonded with the N–N axis in the surface plane and with the molecular plane tilted away from the surface normal by about 30°. This structure provides a simple basis for understanding the facile reaction to adsorbed N2O and O which occurs on heating to 70 to 90 K.

Electronic and structural investigations of technetium compounds by x-ray-absorption spectroscopy

Abstract: X-ray absorption near edge structure spectroscopy has been used to establish the chemical shifts of the technetium K edge in a range of compounds containing Tc in a variety of formal oxidation states. The edge positions span 19.9 eV from Tc metal to NH{sub 4}TcO{sub 4}. Strong correlation between chemical shift and formal oxidation state is observed. Extended X-ray absorption fine structure (EXAFS) spectroscopy of Tc{sub 2}(CO){sub 10} indicates that multiple scattering along the Tc-C-O vector is more important than direct Tc{center_dot}{center_dot}O scattering. TcO{sub 2} is shown by EXAFS to possess a distorted rutile structure with a closest Tc-Tc distance of 2.61 {Angstrom}. This is rationalized in terms of the Goodenough model for bonding in transition metal dioxides.

Structural, morphological, and magnetic study of nanocrystalline cobalt-copper powders synthesized by the polyol process

Abstract: Nanocrystalline CoxCu100−x (4 ⋚ x ⋚ 49 at. %) powders were prepared by the reduction of metal acetates in a polyol. The structure of powders was characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), extended x-ray absorption fine structure (EXAFS) spectroscopy, solid-state nuclear magnetic resonance (NMR) spectroscopy, and vibrating sample magnetometry (VSM). As-synthesized powders were composites consisting of nanoscale crystallites of face-centered cubic (fcc) Cu and metastable face-centered cubic (fcc) Co. Complementary results of XRD, HRTEM, EXAFS, NMR, and VSM confirmed that there was no metastable alloying between Co and Cu. The NMR data also revealed that there was some hexagonal-closed-packed (hcp) Co in the samples. The powders were agglomerated, and consisted of aggregates of nanoscale crystallites of Co and Cu. Upon annealing, the powders with low Co contents showed an increase in both saturation magnetization and coercivity with increasing temperature. The results suggested that during preparation the nucleation of Cu occurred first, and the Cu crystallites served as nuclei for the formation of Co.

Structural characterization of organocuprate reagents. EXAFS spectroscopy and ab initio calculations

Abstract: The cyanocuprate reagents, prepared from CuCN + 7 equiv of organolithium, appear to possess both unique spectroscopic and unique reactivity properties in comparison with the analogous reagents prepared from CuX (X = Cl, Br, I). We have used a combination of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies, together with ab initio structure calculations, to explore the structure of the Cu in these reagents. The XANES measurements support earlier, preliminary EXAFS indications that cyanide is not coordinated to Cu in the so-called `higher-order` cuprate prepared from CuCN + 2BuLi. CuCN{center_dot}2LiCl exists in THF as an oligomer of ...Cu-C=N-Cu... units. The average Cu environment consists of two coordinated cyanides with the possible presence of a third ligand for some of the Cu sites. CuCN + BuLi and CuCN + 2BuLi both contain two-coordinate Cu, with average Cu-C bond lengths of 1.89 and 1.93 A, respectively. One cyanide remains coordinated to Cu in CuCN + BuLi, while both cyanides are displaced in CuCN + 2BuLi. Ab initio structure calculations are consistent with this picture, even to the extent of predicting the correct average Cu bond lengths. The theoretical calculations demonstrate that, in the most stablemore » structure, the cyanide is associated with the alkyl groups that are bound to the Cu. 76 refs., 6 figs., 1 tab.« less

Extended x‐ray absorption fine structure analysis of the difference in local structure of tantalum oxide capacitor films produced by various annealing methods

Abstract: Extended x‐ray absorption fine structure (EXAFS) above the Ta L3 edge on tantalum oxide capacitor films has been measured Tantalum oxide films were prepared by low‐pressure chemical vapor deposition (CVD) using a Ta(OC2H5)5 and O2 gas mixture Four kinds of tantalum oxide films were studied: as‐deposited (amorphous), N2 annealed (crystalline), dry O2 annealed (crystalline), and O2‐plasma annealed (amorphous) From EXAFS analysis, differences in the local structures of tantalum oxide capacitor films, in terms of oxygen deficiency around Ta, were observed in the various annealed films The leakage current characteristics of tantalum oxide capacitors correspond to the differences in the local structures around Ta

X-ray absorption spectroscopy of Ti-containing molecular sieves ETS-10, aluminum-free Ti-β, and TS-1.

Abstract: We report here the first structural characterization by Ti K-edge EXAFS and XANES of ETS-10 and aluminum-free Ti-β molecular sieves. A TS-1 sample was also studied for comparison. The near-edge spectrum of TS-1 revealed an intense pre-edge feature characteristic of Ti atoms in tetrahedral coordination. However, the pre-edge peak for ETS-10 was quite small and shifted in energy, which is consistent with the known octahedral structure surrounding Ti atoms in the material. Analysis of the EXAFS for ETS-10 revealed an average Ti-O interatomic distance of 2.00 +- 0.01 A which is longer than the Ti-O distance (1.95A) in anatase TiO2. For Ti-β, an intense pre-edge peak is present in the near-edge spectrum and the intensity, energy and width of this peak are the same as for TS-1. In addition, the average Ti-O interatomic distance in Ti-β and TS-1 was 1.80 +- 0.01 A, which is significantly shorter than the distance measured in ETS-10. The EXAFS and XANES results indicate that the Ti sites in aluminum-free Ti-β are structurally identical to the tetrahedral sites in TS-1. This conclusion is supported by the similarity of the UV absorption thresholds for the two samples. X-ray absorption spectra were also recorded with methanol or 2-propanol adsorbed on the Ti-β and TS-1 molecular sieves. Alcohol adsorption decreased the intensity and broadened the Ti pre-edge peak for both samples, demonstrating a local chemical interaction with the Ti sites. Methanol adsorption lengthened the average Ti-O bond to 1.83 ± 0.01 A. The similarity of the X-ray absorption spectra for aluminum-free Ti-β and TS-1 in the presence of alcohols suggests that the Ti sites in the two frameworks are chemically indistinguishable for vapor-phase adsorption of alcohols at low concentrations.

The NO2 absorption spectrum. II: Absorption cross-sections at low temperatures in the 400-500 nm region

Abstract: With improved experimental conditions already used for measurements at ambient temperature (Merienneet al., 1994), new values have been found for the absorption cross-sections of NO2 at 240 and 220 K in the 400–500 nm spectral region. Using a better resolution than in previous studies we show that the temperature effect is not negligible and should be taken into account for the optical measurements of atmospheric NO2 amounts by differential absorption methods.

Structure of Fe, Mn-promoted sulfated zirconia catalyst by X-ray and IR absorption spectroscopies

Abstract: The rate of butane isomerization at low temperature is more than two orders of magnitude greater over sulfated zirconia with iron and manganese than over unpromoted sulfated zirconia. Therefore, the bulk and surface structures of the unpromoted and promoted catalysts have been investigated using X-ray absorption spectroscopy at the Zr and Fe K-edges and FTIR spectroscopy of adsorbed pyridine and CO. The Zr—O and Zr—Zr interatomic distances derived from EXAFS indicated that both zirconia catalysts were exclusively tetragonal regardless of composition. In addition, EXAFS revealed the Fe ions did not substitute in the tetragonal structure, but were likely to be present in small clusters or rafts supported on zirconia. The IR spectra, associated with surface sulfate groups, adsorbed pyridine and adsorbed CO, were minimally affected by the promoters indicating that Fe and Mn do not alter the surface properties of the oxides in an obvious manner. Thus, the dramatic effect of promoters on the reaction of butane is likely to be due to extremely low active site densities on the catalysts or the need for two surface sites in the reaction mechanism or both.