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.

Optical Absorption Edge in CdTe: Theoretical

Abstract: Optical absorption in the "edge region" (i.e., for photon energies just below the first exciton peak) due to the creation of direct excitons with the simultaneous absorption of one and two longitudinal optical phonons is calculated by perturbation theory for Wannier excitons formed from the conduction and valence bands around k=0. It is shown that the contributions from the $ng1$ exciton bands, neglected in earlier calculations, are quite important and are required for a quantitative study. The results quantitatively account for the magnitude and for the temperature and energy dependences of the absorption coefficient observed in CdTe using a (average) hole mass of $0.4m$, where $m$ is the free-electron mass. These results strongly support the assignment to CdTe of a "direct" minimum band gap at k=0, contrary to a number of recent contentions (also based on absorption measurements) that the material is "indirect." It is noted that the agreement between the calculated results (including a rough correction for level broadening) and the data extends up to the temperature range at which the absorption coefficient begins to exhibit Urbach's-rule behavior. It is argued from this fact that the mechanism employed in these calculations, as opposed to others recently considered, underlies Urbach's rule in, at least, the compound semiconductors.

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 evolution of an intermetallic Pd–Zn catalyst selective for propane dehydrogenation

Abstract: We report the structural evolution of Pd–Zn alloys in a 3.6% Pd–12% Zn/Al2O3 catalyst which is selective for propane dehydrogenation. High signal-to-noise, in situ synchrotron X-ray diffraction (XRD) was used quantitatively, in addition to in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) and extended X-ray absorption fine structure (EXAFS) to follow the structural changes in the catalyst as a function of reduction temperature. XRD in conjunction with DRIFTS of adsorbed CO indicated that the β1-PdZn intermetallic alloy structure formed at reduction temperatures as low as 230 °C, likely first at the surface, but did not form extensively throughout the bulk until 500 °C which was supported by in situ EXAFS. DRIFTS results suggested there was little change in the surfaces of the nanoparticles above 325 °C. The intermetallic alloy which formed was Pd-rich at all temperatures but became less Pd-rich with increasing reduction temperature as more Zn incorporated into the structure. In addition to the β1-PdZn alloy, a solid solution phase with face-center cubic structure (α-PdZn) was present in the catalyst, also becoming more Zn-rich with increasing reduction temperature.