Extended X-ray absorption fine structure

About: Extended X-ray absorption fine structure is a research topic. Over the lifetime, 10452 publications have been published within this topic receiving 276744 citations.

New Analysis of Oxidation State and Coordination Environment of Copper Ion-Exchanged in ZSM-5 Zeolite

Abstract: The changes of structure and electronic state of copper ion species during the heat treatment of copper ion-exchanged ZSM-5 zeolite (CuZSM-5) as well as the interaction with CO molecules have been investigated by using various spectroscopic techniques such as infrared (IR) and emission spectroscopy (ES), electron spin resonance (ESR), and X-ray absorption fine structure (XAFS) consisting of a XANES (X-ray absorption near edge structure) and an EXAFS (extended X-ray absorption fine structure) and through the measurements of heat of adsorption and adsorption isotherm. About 70% of the divalent copper ions (Cu2+) exchanged in CuZSM-5 were found to be reduced to the monovalent copper ions (Cu+) during the heat treatment at 873 K in vacuo, the latter species having a linear or a planar coordination structure with a coordination number of 2 or 3 with respect to the nearest-neighboring oxygen atoms at a distance of 1.98 A. It was found from the ES data that the Cu+ species strongly interact with CO molecules at ...

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

An extended x‐ray absorption fine structure study of aqueous solutions by employing molecular dynamics simulations

Abstract: Bromine–oxygen radial distribution functions [g(r)] have been calculated by means of molecular dynamics simulations for aqueous solutions of rubidium bromide, 2‐bromopropane and bromoethane. X‐ray absorption spectra at the bromine K edge have been recorded for these solutions. The water contribution to the extended x‐ray absorption fine structure spectra has been calculated starting from the gBr,O(r) distribution function. Fits of the x‐ray absorption spectra have been performed directly on the raw experimental data, allowing the reliability of the g(r) distribution functions to be verified. The agreement between theoretical and experimental spectra is satisfactory. A procedure to improve model g(r) functions on the basis of the short‐range structural information provided by extended x‐ray absorption fine structure data is proposed.

Extended x-ray-absorption fine-structure study of Ag particles isolated in solid argon

Abstract: Extended x-ray-absorption fine structure was used to determine the nearest-neighbor distance for silver particles isolated in solid argon. The silver particles were produced in the gas phase with the use of the gas-aggregation technique. In this technique the metal atoms are evaporated in an argon atmosphere in the pressure range 0.1-10 Torr; a cooling of the metal atoms takes place through collision with cold gas atoms. The metal aggregates are then transported by the gas stream through an aperture into a liquid-helium cryopump, where most of the gas is condensed. The metal flow rate is monitored by a quartz oscillator. The size of the metal clusters can be selected by changing the argon pressure, density of the metal atom vapors, and geometric configuration of the cell. The size distribution was determined by introducing a sample holder into the molecular beam, sampling an atomic quantity of approximately ${10}^{15}$ atoms/${\mathrm{cm}}^{2}$, and evaluating the corresponding electron micrographs. Particles with average sizes between 25 and 130 \AA{} were studied. A careful analysis of the data was performed with a thin silver foil at 78 K as a standard. No remarkable effects were observed in the near-edge region of the $K$ edge, but there was a noticeable contraction of the nearest-neighbor distance. This contraction can be explained if we assume the presence of surface stresses. There is good agreement between the observed contraction and the calculated values.