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.

X-ray diffraction and EXAFS studies of silicate glasses containing Mg, Ca and Ba atoms

Abstract: Local structures around Si, Mg, Ca and Ba atoms in MgSiO 3 , CaSiO 3 , and CaBaSi 2 O 6 glasses were investigated by X-ray diffraction and EXAFS (extended X-ray absorption fine structure) measurements. From these results, the dependence of the size of modifier ions on glass structures is discussed. The SiO interatomic distances of these three glasses are in the range of 0.162 to 0.164 nm and the coordination numbers of Si atoms are estimated to be about 4. The CaO interatomic distances of CaSiO 3 and CaBaSi 2 O 6 glasses are in the range of 0.242 to 0.249 nm and their coordination numbers of Ca atoms are estimated to be about 6. The MgO interatomic distances in MgSiO 3 glass are 0.204 nm and almost equal to those (0.206 nm) in CaMgSi 2 O 6 glass. The BaO interatomic distances in CaBaSi 2 O 6 glass are 0.27 nm. The Si-, Mg- and CaO interatomic distances and the coordination numbers of Si, Mg and Ca atoms in alkaline earth silicate glasses have almost the same values. However, the Debye-Waller factors for CaO pairs and the SiOSi angles become small with increasing size of modifier ions. Comparing the local structures of these glasses with those of the corresponding crystals, the MgO and BaO interatomic distances are shorter than those of the corresponding crystals while the CaO distances and the coordination numbers of Ca atoms in the glasses are close to those of wollastonite and parawollastonite crystals. These differences of local structures are caused by the difference of space occupied by modifier ions in glasses and crystals. Namely, as compared with Mg and Ba atoms, Ca atoms probably have suitable atomic radius for the occupation of the space of SiO 4 network of MSiO 3 (M: Mg, Ca and Ba) composition glasses.

Structure of the water ice surface studied by x-ray absorption spectroscopy at the O K-edge

Abstract: Vapor-deposited H2O ice films grown between 38 and 150 K under ultrahigh vacuum conditions have been investigated using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy at the oxygen K-edge, in conventional mode—which is bulk sensitive-, and using the photon-stimulated desorption mode (PSD-NEXAFS), which is surface sensitive. By recording simultaneously those two signals, we have evidenced the differences between the surface and bulk electronic and atomic structures, for both amorphous porous ice condensed at 40 K and crystalline ice condensed at 150 K. We have also followed the bulk and surface evolutions of an amorphous ice film annealed from 38 to 147 K. A steep change in the local atomic structure of the bulk is observed, likely related to the high-density amorphous ice→low-density amorphous ice phase transition between 38 and 55 K. We have shown that the surface of crystalline ice is well ordered, but this order is different from that of the bulk. We have evidenced that the H2O–H2O int...

Structures of the cuprous-thiolate clusters of the Mac1 and Ace1 transcriptional activators.

Abstract: X-ray absorption spectroscopy on the minimal copper-regulatory domains of the two copper-regulated transcription factors (Ace1 and Mac1) in Saccharomyces cerevisiae revealed the presence of a remarkably similar polycopper cluster in both proteins. The Cu-regulatory switch motif of Mac1 consisting of the C-terminal first Cys-rich motif, designated the C1 domain, binds four Cu(I) ions as does the Cu-regulatory domain of Ace1. The four Cu(I) ions are bound to each molecule in trigonal geometry. An extended X-ray absorption fine structure (EXAFS) arising from outer-shell Cu···Cu interactions at 2.7 and 2.9 A was apparent in each Cu(I) complex indicative of a polycopper cluster. The intensity of the 2.9 A Cu···Cu backscatter peak, apparently diminished by partial cancellation, dominates the EXAFS. The results suggest that CuAce1 and CuMac1(C1) contain somewhat distorted forms of a known [Cu4−S6] cage in which a core of Cu atoms forming an approximate tetrahedron is bound by bridging thiolates above each of the...

Ultraviolet to near‐infrared absorption spectrum of carbon dioxide ice from 0.174 to 1.8 μm

Abstract: [1] A laboratory experiment was devised to measure transmission at fine spectral resolution through thick, high-quality samples of CO2 ice over an extended wavelength range. The absorption coefficient throughout the ultraviolet and near-infrared spectral ranges 0.174–1.8 μm (5555–57,470 cm−1 in wave number) is presented here. CO2 ice samples were grown at a temperature of 150 K, typical of the Martian polar caps. The path length of the samples varied from 1.6 to 107.5 mm, allowing the measurement of absorption from <0.1 to 4000 m−1. The experiment used both a grating monochromator (with spectral resolution 0.1–0.3 nm) and a Fourier transform spectrometer (with an effective resolution of <1.0 cm−1). The transmission data for five thicknesses are used to estimate both the scattering losses for each sample and the absorption coefficient at each wavelength. The uncertainty in the most transparent wavelength regions (<10 m−1) is due to scattering extinction. Measurement noise and data scatter produce significant uncertainty only where absorption coefficients exceed 1000 m−1. Between 1.0 and 1.8 μm there are several weak to moderate absorption lines. Only an upper limit to the absorption can be determined in many places; e.g., the absorption from ∼0.25 to 1.0 μm is below the detection limit. The estimated visible absorption, ∼10–2 m−1, is a factor of 1000 smaller than the values reported by Egan and Spagnolo, which have been used previously to compute albedos of CO2 snow. The new results should be useful for studies of the seasonal polar caps of Mars.

Diphosphine-Protected Au22 Nanoclusters on Oxide Supports Are Active for Gas-Phase Catalysis without Ligand Removal.

Abstract: Investigation of atomically precise Au nanoclusters provides a route to understand the roles of coordination, size, and ligand effects on Au catalysis. Herein, we explored the catalytic behavior of a newly synthesized Au22(L8)6 nanocluster (L = 1,8-bis(diphenylphosphino) octane) with in situ uncoordinated Au sites supported on TiO2, CeO2, and Al2O3. Stability of the supported Au22 nanoclusters was probed structurally by in situ extended X-ray absorption fine structure (EXAFS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and their ability to adsorb and oxidize CO was investigated by IR absorption spectroscopy and a temperature-programmed flow reaction. Low-temperature CO oxidation activity was observed for the supported pristine Au22(L8)6 nanoclusters without ligand removal. Density functional theory (DFT) calculations confirmed that the eight uncoordinated Au sites in the intact Au22(L8)6 nanoclusters can chemisorb both CO and O2. Use of isotopically labeled O2...