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.

First-principles study of the band structure and optical absorption of CuGaS2

Abstract: CuGaS${}_{2}$ is the most promising chalcopyrite host for intermediate-band thin-film solar cells. Standard Kohn-Sham density functional theory fails in describing the band structure of chalcopyrite materials, due to the strong underestimation of the band gap and the poor description of $p$-$d$ hybridization, which makes it inadvisable to use this approach to study the states in the gap induced by doping. We used a state-of-the-art restricted self-consistent $GW$ approach to determine the electronic states of CuGaS${}_{2}$: in the energy range of interest for optical absorption, the $GW$ corrections shift the Kohn-Sham bands almost rigidly, as we proved through analysis of the effective masses, bandwidths, and relative position of the conduction energy valleys. Furthermore, starting from the $GW$ quasiparticle bands, we calculated optical absorption spectra using different approximations. We show that the time-dependent density functional theory can be an efficient alternative to the solution of the Bethe-Salpeter equation when the exchange-correlation kernels derived from the Bethe-Salpeter equation are employed. This conclusion is important for further studies of optical properties of supercells including dopants.

The structure and reactivity of chemisorbed aromatics: Spectroscopic studies of benzene on Mo(110)

Abstract: The reactions of benzene on Mo(110) have been investigated under ultrahigh vacuum conditions using temperature programmed reaction spectroscopy (TPRS), x‐ray photoelectron spectroscopy (XPS), and near‐edge x‐ray absorption fine structure (NEXAFS) measurements. Benzene undergoes competing decomposition and desorption processes during temperature programmed reaction; saturation exposures of C6H6 made at a crystal temperature of 120 K yield a small amount of benzene desorption at 360 K as well as substantial H2 evolution at 400 and 530 K. Additionally, a trio of molecular benzene desorptions attributed to weakly bound and multilayer states is observed below 200 K. Isotopic exchange experiments demonstrate that the 360 K molecular desorption arises from intact chemisorbed benzene. For surface temperatures up to 300 K, carbon 1s photoelectron spectra show only a single photoemission peak at a binding energy of 284.0 eV, attributed to molecular benzene. Above 300 K, increasing amounts of atomic carbon are obser...

Structure of supported pdau clusters determined by x-ray absorption spectroscopy

Abstract: The extended X-ray absorption fine structure (EXAFS) above the Pd K and Au L[sub III] edges and the X-ray absorption near edge structure (XANES) associated with the Au L[sub III] edge were used to study the structure and local electronic environment of PdAu bimetallic clusters supported on silica. Four samples with total metal loadings of approximately 100 [mu]mol g[sup [minus]1] and varying PdAu compositions were investigated. Transmission electron microscopy performed on a bimetallic sample (Pd:Au mole ratio equal to 60:40) exposed to air failed to detect any metal particles, indicating that their average diameter was less than 1 nm, which corresponds to [approximately] 100% metal exposed. The EXAFS for bimetallic clusters (Pd:Au = 60:40) in vacuum at 77 K shows that Pd atoms decorate a core of Au atoms. The Pd-Pd interatomic distance in the bimetallic sample is the same as in bulk Pd, whereas the Au-Au interatomic distance is 3% shorter than that in bulk Au. The XANES associated with the Au L[sub III] edge indicates that Au has an atomic-like electronic structure when alloyed with Pd, which is due to Au atoms having fewer like nearest neighbors in the clusters compared to bulk Au. 38 refs., 8 figs.,more » 3 tabs.« less

The structure of U6+ sorption complexes on vermiculite and hydrobiotite

Abstract: The sorption of the uranyl oxo-cation (UO22+)at different types of binding sites on layer silicate mineral surfaces was investigated. Well-characterized samples of vermiculite and hydrobiotite were exposed to aqueous uranyl under conditions designed to promote surface sorption either at fixed charge ionexchange sites or at amphoteric surface hydroxyl sites. The local structure of uranium in the sorption samples was directly measured using uranium L3-edge extended X-ray absorption fine structure (EXAFS). Polarized L1- and L3-edge X-ray absorption near-edge structure (XANES) measurements were used to characterize the orientation of uranyl groups in layered samples. X-ray diffraction (XRD) measurements of interlayer spacings were used to assess the effects of ion-exchange and dehydration upon the mineral structure. The most significant findings are: (1) Under conditions which greatly favor ion-exchange sorption mechanisms, uranyl retains a symmetric local structure suggestive of an outer-sphere complex, with a preferred orientation of the uranyl axis parallel to the mineral layers; (2) Upon dehydration, the ionexchange complexes adopt a less symmetric structure, consistent with an inner-sphere complex, with less pronounced orientation of the uranyl axis; and (3) For conditions which favor sorption at surface hydroxyl sites, uranyl has a highly distorted equatorial shell, indicative of stronger equatorial ligation, and the detection of a neighboring U atom suggests the formation of surface precipitates and/or oligomeric complexes.

Hydration properties of the bromide aqua ion: the interplay of first principle and classical molecular dynamics, and X-ray absorption spectroscopy.

Abstract: The hydration properties of the bromide aqua ion have been investigated using state of the art density functional theory (DFT) based molecular dynamics with dispersion-corrected atom-centered pseudopotentials for water and classical molecular dynamics simulations. The reliability of the theoretical results has been assessed by comparing the attained structural results with the extended X-ray absorption fine structure (EXAFS) experimental data. The EXAFS technique is mainly sensitive to short distances around the bromine atom, and it is a direct probe of the local solvation structure. The comparison shows that the DFT simulation delivers a good description of the EXAFS experimental signal, while classical simulation performs poorly. The main reason behind this is the neglect of polarization effects in the classical ion−water interaction potentials. By taking advantage of the reliable information on the Br− local hydration structure it has been possible to highlight the contribution of hydrogen atoms to the...