Full Multiple-scattering Analysis Research Articles

Manganese Oxides: Parallels between Abiotic and Biotic Structures

A large number of microorganisms are responsible for the oxidation of Mn(2+)((aq)) to insoluble Mn(3+/4+) oxides (MnO(x)()) in natural aquatic systems. This paper reports the structure of the biogenic MnO(x)(), including a quantitative analysis of cation vacancies, formed by the freshwater bacterium Leptothrix discophora SP6 (SP6-MnO(x)()). The structure and the morphology of SP6-MnO(x)() were characterized by transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), including full multiple-scattering analysis, and powder X-ray diffraction (XRD). The biogenic precipitate consists of nanoparticles that are approximately 10 nm by 100 nm in dimension with a fibrillar morphology that resembles twisted sheets. The results dem-onstrate that this biogenic MnO(x)() is composed of sheets of edge-sharing of Mn(4+)O(6) octahedra that form layers. The detailed analysis of the EXAFS spectra indicate that 12 +/- 4% of the Mn(4+) layer cation sites in SP6-MnO(x)() are vacant, whereas the analysis of the XANES suggests that the average oxidation state of Mn is 3.8 +/- 0.3. Therefore, the average chemical formula of SP6-MnO(x)() is M(n)()(+)(y)()Mn(3+)(0.12)[ square(0.12)Mn(4+)(0.88)]O(2).zH(2)O, where M(n)()(+)(y)() represents hydrated interlayer cations, square(0.12) represents Mn(4+) cation vacancies within the layer, and Mn(3+)(0.12) represents hydrated cations that occupy sites above/below these cation vacancies.

Light scattering from a randomly occupied optical lattice. II. The multiple scattering problem.

In this paper, we study the problem of multiple scattering of light from a randomly occupied optical lattice, thereby extending the first-order Born analysis of the previous paper. A full multiple-scattering analysis is essential to a complete understanding of the nature of light propagation inside a medium. Our calculations show that the incident wave, when resonant with the atomic medium, is rapidly extinguished due to multiple scattering. The decay constant depends critically on the incident wavelength, the lattice constant, the average number density of atoms, and their polarizability. Both the Bragg scattering amplitudes and directions are modified as a result of multiple scattering. Because of the random site occupation of an otherwise regular lattice structure, a coherent enhancement of the scattering cross section is also predicted to occur along a discrete set of directions that are related to the strictly backward direction by reciprocal lattice vectors.

Extracting Dynamic Information from EXAFS: Simultaneous Analysis of Multiple Temperature-Dependent Data

A new approach to the extraction of dynamic information from extended X-ray absorption fine-structure (EXAFS) spectra has been developed. With this method, a complete set of temperature-dependent spectra are fit simultaneously to one of a variety of pair-distribution functions. Distributions are calculated in r-space using the appropriate absorber-scatterer pair potential. The temperature-dependent EXAFS spectra are calculated by summing k-space models over a range of distances and angles weighted according to the relative contribution of each geometry to the distribution. This approach allows refinement of data using a full multiple-scattering analysis with only modest computational time.

Electronic structure ofNi3Al andNiAl3alloys:mX-ray-absorption fine-structure analysis

X-ray-absorption fine structure (XAFS) above the Ni K edge in Ni{sub 3}Al and NiAl{sub 3} alloys has been measured and theoretical full multiple-scattering analysis of these data have been done. The theoretical XAFS are found to be in agreement with experimental data. The XAFS of Ni{sub 3}Al and NiAl{sub 3} alloys are rather different. Since the dipole transition matrix element is not a very sharp function of the energy the experimental XAFS reflects the averaged in space partial Ni p unoccupied states in the conduction bands of the Ni{sub 3}Al and NiAl{sub 3} alloys, showing changes in the electronic structure going from Ni{sub 3}Al to NiAl{sub 3} alloy. Theoretical partial density of states curves calculated along the axis parallel to the c vector differ from the partial density of states curves calculated in the {ital ab} plane for both alloys. {copyright} {ital 1997} {ital The American Physical Society}

Spin-dependent Mn K-edge XANES of MnO and MnF2: Full multiple-scattering analysis.

We have performed the self-consistent full multiple-scattering analysis of the spin-dependent Mn K-edge XANES (x-ray-absorption near-edge structure) of MnO and ${\mathrm{MnF}}_{2}$. Our approach is based on successive self-consistent-field spin-polarized calculations of small clusters and full multiple-scattering calculations of XANES within large size clusters. We have considered a cluster of nine shells of surrounding atoms for the MnO case and five shells for the ${\mathrm{MnF}}_{2}$ case. We have succeeded in calculating the spin-dependent XANES over a 40-eV energy range in the continuum above the absorption threshold in agreement with experimental data. The results show that both the spin-dependent transition matrix element and Mn p partial density of states contribute to the spin-dependent Mn K-edge XANES of MnO and ${\mathrm{MnF}}_{2}$.

Spin-dependent Mn XANES of MnO: theoretical analysis

Recently, a new high resolution spin-dependent XANES technique has been developed and its application for MnO has displayed a well-pronounced difference in the energy position and relative intensity of XANES peaks, corresponding to two different spin configurations of the MnO electronic subsystem. Here we report the full multiple-scattering analysis of the spin-dependent Mn K-edge XANES of MnO. We succeed in calculating spin-dependent XANES over a large energy range in the continuum with the help of a self-consistent spin-polarized method. This approach will be effective for the theoretical analysis of spin-dependent X-ray absorption in complex and disordered matter, where other approaches using band structure theory cannot be used.

Spin-dependent Mn X-ray absorption near-edge structure of MnF2: full multiple-scattering analysis

The self-consistent full multiple-scattering analysis of the spin-dependent Mn K-edge X-ray absorption near-edge structure (XANES) of MnF2 has been carried out. We succeeded in calculating the spin-dependent XANES over a 40 eV energy range in the continuum above the absorption threshold with the help of a self-consistent spin-polarized method in agreement with experimental data. We show that both the spin-dependent transition matrix element and the Mn p partial density of states contribute to the spin-dependent Mn K-edge XANES of MnF2.

X‐Ray Absorption Fine Structure Investigation of the High Pressure Phase of KBr and RbCl. Full Multiple Scattering Analysis

AbstractX‐ray absorption near edge structure (XANES) of the Br and Rb K edge of the high pressure (B2) phase of KBr and RbCl is analyzed. A full multiple scattering method is used to investigate the conduction band structure of KBr and RbCl. Theoretical K‐edge absorption spectra obtained over a large energy interval are used to analyze the experimental data. As in the case of the low pressure (B1) phase, XANES is found to be determined by multiple scattering of photoelectrons in a large‐size cluster (30 atoms or more). For the energy interval up to 50 eV above the main edge, the single scattering approximation seems to be less satisfactory. A comparative analysis of XANES of the two compounds in two different crystal phases makes it possible to conclude on the role of the local symmetry around the absorbing site and the characteristics of the surrounding atoms in the formation of XANES and conduction band of the compounds investigated.

X-ray absorption fine-structure investigation of the ionic compounds NaBr, KBr and RbCl: full multiple-scattering analysis

The x-ray absorption near-edge structures (XANESs) of the Br K edge in NaBr and KBr and the Rb K edge in RbCl have been analysed. In spite of the fact that these compounds have the same type of crystal lattice (NaCl type) the published experimental spectra show different structures. We have used a full multiple-scattering method to investigate the conduction band structures of NaBr, KBr and RbCl. The partial densities of Br and Rb p states in the conduction bands were obtained over a large energy interval for both the ground and the excited states. The latter were used to analyse the experimental data. In contrast with our recent study of AgBr, the Br K XANESs in NaBr and KBr and the Rb K XANES in RbCl are found to be determined by the multiple scattering of photoelectrons in a large cluster (30 or more atoms). We also show that, for the energy interval up to 80 eV above the main edge, the single-scattering approximation in the present study seems to be less satisfactory.

Polarization Dependence of XANES of α-Quartz: Experiments and Full Multiple-Scattering Calculations

The experimental spectra of α-quartz present a strong dichroic dependance at both silicon and oxygen edges. A full multiple-scattering analysis is presented at both edges.

Quantitative multiple-scattering analysis of near-edge x-ray-absorption fine structure: c(22)O on Cu(100).

The structure of c(2\ifmmode\times\else\texttimes\fi{}2)O on Cu(100) is investigated with a full multiple-scattering analysis of the near-edge x-ray-absorption fine structure (NEXAFS). Various adsorption sites and bond lengths are tested, with the oxygen overlayer found to occupy fourfold hollow sites 0.7 A\r{} above the Cu surface plane, in agreement with recent surface extended x-ray-absorption fine-structure (SEXAFS) work on the same sample. Our study provides the first demonstration that a quantitatively accurate NEXAFS analysis is attainable with low-noise high-resolution data, and opens up new possibilities for surface analysis for those cases where SEXAFS is not applicable.