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X-ray Absorption Fine Structure - XAFS14: 14th International Conference on X-ray Absorption Fine Structure
No full text14th International Conference on X-ray Absorption Fine Structure, Camerino, Italy, 26-31 July 200
Atomic Background In X-ray-absorption Spectra of 5th-period Elements - Evidence For Double-electron Excitation Edges
No full textA systematic x-ray absorption study to detect atomic background features at the K edges of fifth-period elements is reported. Samples of MoCx (Z = 42) and pure Rh (Z = 45), Pd (Z = 46), Ag (Z = 47), In (Z = 49), and Sn (Z = 50) are measured at high temperature, mostly in the liquid phase, to reduce the intensity of the structural extended x-ray absorption fine structure (EXAFS) contribution. Main anomalies, showing a regular trend as a function of Z, are identified in the region around 100 eV and in the range 200-800 eV above an edge. They are assigned to the opening of absorption channels creating double-hole configurations such as [1s4p], [1s, 3d], and [1s3p]. Differences of self-consistent calculations in the Dirac-Fock approximation have been performed and the dominant transitions unambiguously identified. The results are relevant for the improvement of the EXAFS analysis at these edges
Local g(r) properties in liquids probed by high-temperature EXAFS
No full textRecent experimental advances in high-temperature X-ray absorption spectroscopy (XAS) have stimulated applications to the field of liquid metals and semiconductors. It is now possible to reach the 3000 K temperature range under vacuum or controlled atmosphere. Accurate XAS experiments under high-temperature conditions have been performed on crystalline, molten and highly supercooled metals. Initial applications included liquid and supercooled Sn, Ge, Cu, Ag, and Rh at various temperatures. Reliable short-range pair distribution g(r) functions have been determined using ab initio multiple-scattering calculations. The results are directly comparable with diffraction experiments or molecular dynamic simulations, XAS is found to be particularly sensitive to the first rise of the g(r) and can be considered a reliable complementary technique for studying the short-range structure of liquids. A clear trend in the shape of g(r) as a function of temperature is usually found. The application of this technique to liquid Sn over a large temperature range is reported
Investigating local three-body correlations by means of a novel XAS data-analysis method
No full textno inf
X-ray-absorption fine structure in embedded atoms - Comment
No full textWe illustrate that the possible existence of an oscillatory atomic x-fay-absorption background (AXAFS) as reported by Rehr et al
The 14th International Conference on X-ray Absorption Fine Structure (XAFS14)
No full textThis is the preface of the proceedings volume of the 14th International Conference on X-ray Absorption Fine Structure (XAFS14) held in Camerino, Italy, July 26-31, 2009
Evidence For [2p(s)4f] Multielectron Resonances In X-ray-absorption Spectra of 6th-period Elements
No full textClear evidence for multielectron excitation channels involving 2p or 2s and 4f electrons has been discovered in x-ray-absorption L-edge spectra of several sixth-period elements. Measurements on liquid Pb and Bi, as well as in PbO, Pb(CH3CO2)2, and Bi2O3 compounds, are reported and compared with previous data on vapor Hg. Onset energies of the double-electron excitation channels are found to be in agreement with previously published self-consistent calculations. A weaker feature, which has been observed in several spectra, is assigned to a triple-electron excitation involving two strongly correlated 4f electrons. The importance of these findings for the extended x-ray-absorption fine structure structural analysis of sixth-period elements is discussed
Short-range Order In Crystalline, Amorphous, Liquid, and Supercooled Germanium Probed By X-ray-absorption Spectroscopy
No full textA detailed experimental investigation of the short-range structural properties in condensed phases of germanium is presented. X-ray-absorption measurements at the Ge K edge have been collected in a wide temperature range for different samples. Polycrystalline c-Ge was measured at 77, 296, 450, 620, 782, 920, and 1110 K, close to the Ge melting-point temperature Tm=1210.4 K. Evaporated amorphous Ge was measured at 297 K. Eight independent measurements for liquid germanium have been collected from about 950 K in highly supercooled conditions up to about 1600 K. The spectra show a remarkable temperature trend. By comparison, previous diffraction measurements on l-Ge were limited to two narrow temperature regions only, either above Tm or around 1500 K, and no measurements in the supercooled liquid region existed. Data analysis has been performed with the GNXAS approach and account has been taken for the presence of double-excitation channels involving 3d and 3p electrons in addition to the 1s. The c-Ge structural results are found in excellent agreement with the known properties. The expansion of the average bond length R is in agreement with thermal expansion data. Mean-square vibrational amplitudes are in excellent agreement with both previous measurements and calculations in the harmonic approximation. The analysis of the signal in liquid Ge has been performed using a technique that allows to extract information on the radial distribution function g(r) directly comparable with molecular dynamics (MD) simulations or previous diffraction determinations.
A regular trend is observed in the intensity of the first g(r) peak that decreases from 2.3 to 950 K to about 1.8 at 1610 K. At the same time a widening of the peak and a shift of the rising short distance edge is clearly detected. The data are in excellent agreement with diffraction measurements and recent ab initio MD results by Kresse and Hafner [Phys. Rev. B 49, 14 251 (1994)]. The general relevance of these findings, in connection with the possibility to obtain structural information for liquid systems complementary to that contained in diffraction measurements, is addressed
Local Correlations In Liquid and Supercooled Gallium Probed By X-ray-absorption Spectroscopy
No full textAn investigation on the local structure of liquid and supercooled pure gallium at several temperatures is presented. Accurate EXAFS (Extended X-ray Absorption Fine Structure) spectra have been analysed by means of ab initio calculations of the multiple-scattering signals associated with two-atom and three-atom configurations. The extracted g2(r) first-neighbour distribution of the liquid phase was found to be in excellent agreement with diffraction measurements. Clear evidence of nearly covalent bonds has been found in highly supercooled gallium where the asymmetric first-neighbour distribution is split into two resolved peaks. Analysis of the three-atom g3(r1, r2, theta) distribution has shown evidence for nearly-equilateral and nearly-orthogonal configurations
Development of An Oven For X-ray-absorption Measurements Under Extremely High-temperature Conditions
No full textWe developed a high-temperature oven suitable for X-ray absorption studies that can operate from room temperature up to about 3000 K under high vacuum conditions or possibly controlled atmosphere. The main characteristic of the oven is its simplicity and quick operation. Typical heating and cooling rates can be faster than 100 K/s. This is obtained by heating only a very small graphite crucible inside a glass vessel. The sample size is compatible with typical X-ray beams available and very good temperature homogeneity can be obtained. The oven can be useful to study structural properties of highly anharmonic solids, liquid elements and alloys, as well as to probe the structural effects of temperature-induced chemical reactions. Preliminary measurements on liquid germanium, liquid platinum and molybdenum carbide are presented
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