The Experts below are selected from a list of 297 Experts worldwide ranked by ideXlab platform
John J. Rehr - One of the best experts on this subject based on the ideXlab platform.
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High-throughput computational X-Ray Absorption spectroscopy
Scientific Data, 2018Co-Authors: Kiran Mathew, John J. Rehr, Chen Zheng, Donald Winston, Chi Chen, Alan Dozier, Shyue Ping Ong, Kristin A. PerssonAbstract:Design Type(s) database creation objective Measurement Type(s) near-edge X-Ray Absorption fine structure spectrum Technology Type(s) computational modeling technique Factor Type(s) chemical entity Machine-accessible metadata file describing the reported data (ISA-Tab format) X-Ray Absorption spectroscopy (XAS) is a widely-used materials characterization technique. In this work we present a database of computed XAS spectra, using the Green's formulation of the multiple scattering theory implemented in the FEFF code. With more than 500,000 K-edge X-Ray Absorption near edge (XANES) spectra for more than 40,000 unique materials, this database constitutes the largest eXisting collection of computed XAS spectra to date. The data is openly distributed via the Materials Project, enabling researchers across the world to access it for free and use it for comparisons with eXperiments and further analysis.
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Hole counts from X-Ray Absorption spectra.
Journal of Synchrotron Radiation, 2001Co-Authors: A. L. Ankudinov, Alexey I. Nesvizhskii, John J. RehrAbstract:The interpretation of X-Ray Absorption spectra in terms of electronic structure has long been of interest. Hole counts derived from such spectra are often interpreted in terms of free-atom occupation numbers or Mulliken counts. It is shown here, however, that renormalized-atom (RA) and cellular counts are better choices to characterize the configuration of occupied electron states in molecules and condensed matter. A projection-operator approach is introduced to subtract delocalized states and to determine quantitatively such hole counts from X-Ray Absorption spectra. The described approach is based on multiple-scattering theory (MST) and on atomic calculations of a smooth transformation relating the X-Ray Absorption spectra to local projected densities of states (LDOS). Theoretical tests for the s and d electrons in transition metals show that the approach works well.
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Multiple-scattering calculations of X-Ray-Absorption spectra
Physical review. B Condensed matter, 1995Co-Authors: S. I. Zabinsky, A. L. Ankudinov, John J. Rehr, Robert C. AlbersAbstract:A high-order multiple-scattering (MS) approach to the calculation of polarized X-Ray-Absorption spectra, which includes both X-Ray-Absorption fine structure and X-Ray-Absorption near-edge structure, is presented. Efficient calculations in arbitrary systems are carried out by using a curved-wave MS path formalism that ignores negligible paths, and has an energy-dependent self-energy and MS Debye-Waller factors. Embedded-atom background Absorption calculations on an absolute energy scale are included. The theory is illustrated for metallic Cu, Cd, and Pt. For these cases the MS eXpansion is found to converge to within typical eXperimental accuracy, both to eXperiment and to full MS theories (e.g., band structure), by using only a few dozen important paths, which are primarily single-scattering, focusing, linear, and triangular.
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Sum rules for polarization-dependent X-Ray Absorption
Physical review. B Condensed matter, 1995Co-Authors: A. L. Ankudinov, John J. RehrAbstract:A complete set of sum rules is obtained for polarization-dependent X-Ray-Absorption fine structure and X-Ray circular magnetic dichroism (CMD), analogous to those for CMD derived by Thole et al. These sum rules relate X-Ray-Absorption coefficients to the ground-state eXpectation values of various operators. Problems with applying these sum rules are discussed.
Frank M. F. De Groot - One of the best experts on this subject based on the ideXlab platform.
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OXygen K-edge X-Ray Absorption Spectra
Chemical reviews, 2020Co-Authors: Federica Frati, Myrtille O. J. Y. Hunault, Frank M. F. De GrootAbstract:We review oXygen K-edge X-Ray Absorption spectra of both molecules and solids. We start with an overview of the main eXperimental aspects of oXygen K-edge X-Ray Absorption measurements including X-Ray sources, monochromators, and detection schemes. Many recent oXygen K-edge studies combine X-Ray Absorption with time and spatially resolved measurements and/or operando conditions. The main theoretical and conceptual approXimations for the simulation of oXygen K-edges are discussed in the Theory section. We subsequently discuss oXygen atoms and ions, binary molecules, water, and larger molecules containing oXygen, including biomolecular systems. The largest part of the review deals with the eXperimental results for solid oXides, starting from s- and p-electron oXides. EXamples of theoretical simulations for these oXides are introduced in order to show how accurate a DFT description can be in the case of s and p electron overlap. We discuss the general analysis of the 3d transition metal oXides including discussions of the crystal field effect and the effects and trends in oXidation state and covalency. In addition to the general concepts, we give a systematic overview of the oXygen K-edges element by element, for the s-, p-, d-, and f-electron systems.
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Invisible structures in the X-Ray Absorption spectra of actinides
Journal of Electron Spectroscopy and Related Phenomena, 2014Co-Authors: Kristina O. Kvashnina, Frank M. F. De GrootAbstract:The X-Ray Absorption spectra of actinides are discussed with an emphasis on the fundamental effects that influence their spectral shape, including atomic multiplet theory, charge transfer theory and crystal field theory. Many actinide spectra consist of a single peak and it is shown that the use of resonant inelastic X-Ray emission spectra (RIXS) has the potential to reveal many new features in the X-Ray Absorption spectra of actinides. The new range of RIXS beamlines will allow the determination of new structures in the X-Ray Absorption spectra that have been hitherto invisible. This has the potential to become an important tool in the determination of the electronic structure of actinides.
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High-Resolution X-Ray Emission and X-Ray Absorption Spectroscopy
Chemical reviews, 2001Co-Authors: Frank M. F. De GrootAbstract:In this review, high-resolution X-Ray emission and X-Ray Absorption spectroscopy will be discussed. The focus is on the 3d transition-metal systems. To understand high-resolution X-Ray emission and reso-nant X-Ray emission, it is first necessary to spend some time discussing the X-Ray Absorption process. Section II discusses 1s X-Ray Absorption, i.e., the K edges, and section III deals with 2p X-Ray Absorption, the L edges. X-Ray emission is discussed in, respectively, the L edges. X-Ray emission is discussed in, respec-tively, and section V on 2p3s and 2p3d X-Ray emission. Section VI focuses on magnetic dichroism effects, and in section VII selective X-Ray Absorption eXperiments are discussed. To limit the scope of this review paper, many related topics (for eXample, EELS, XPS, and resonant photoemission, phonon-oriented inelastic X-Ray scat-tering, and X-Ray microscopy) will not be discussed. In addition, many aspects of X-Ray Absorption, such as reflection eXperiments, diffraction Absorption fine structure, and related eXperiments, will remain untouched. EXAFS will be discussed very briefly, and its X-Ray emission analogue EXEFS 69,71 will not be discussed.
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X-Ray Absorption and dichroism of transition metal compounds
The 17th international conference on x-ray and inner-shell processes, 1997Co-Authors: Frank M. F. De GrootAbstract:The analysis of X-Ray Absorption spectra to determine the electronic and magnetic structure of transition metal compounds is discussed. The models to describe the ground state of transition metal compounds (single-particle, impurity, crystal field) are introduced. Some basic aspects of the interaction of X-Rays with matter are recapitulated and the description of X-Ray Absorption is separated into single-particle models for the 1s edges and multiplet models for the 2p edges. Magnetic circular dichroism is introduced and the siX Thole sum rules are discussed. The complications and eXperimental problems of the sum rules are outlined. The last section briefly mentions some aspects of resonance studies, for which a detailed knowledge of X-Ray Absorption is crucial.
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Theory of X-Ray Absorption
Synchrotron Radiation: Selected Experiments in Condensed Matter Physics, 1991Co-Authors: Frank M. F. De GrootAbstract:This contribution concerns the interpretation of X-Ray Absorption spectra for the determination of the electronic structure. This subject has gained much impetus in recent years as a direct consequence of the eXperimental progress in the field of soft X-Ray monochromators, like the SX700 plane-grating-monochromators [1] and the DRAGON mono chromat or, based on a cylindrical-grating [2]; both monochromators reaching up to 1:10.000 resolution in the soft X-Ray range. This stimulated the development of many new fields one of which is 2p X-Ray Absorption (L 2,3-edges) of the 3d transition-metal compounds. The high resolution, combined with the relative long lifetime of the eXcited states, result in eXtremely sharp spectra with compleX shapes. In the process of eXplaining these shapes it was immediately clear that the role of the core-hole was of central importance, like in the case of 3d X-Ray Absorption spectra (or M 4,5-edges) of the Rare-Earths. The eXperiments on the M 4,5-edges in the siXties [3] were, for some early Rare-Earths, partially eXplained with an atomic multiplet approach, developed in the early seventies [4]. This approach was improved and generalized to all Rare-Earths by Thole et al. and good agreement was found with eXperiment [5]. Although the atomic multiplet results show some likeness with the L 2,3-edges of the 3d transition-metal compounds, solid state effects modify the spectra to such eXtend that detailed analysis calls for a change in the approach. In the following we will in short give an overview of the some models to simulate the X-Ray Absorption cross section.
A. L. Ankudinov - One of the best experts on this subject based on the ideXlab platform.
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Hole counts from X-Ray Absorption spectra.
Journal of Synchrotron Radiation, 2001Co-Authors: A. L. Ankudinov, Alexey I. Nesvizhskii, John J. RehrAbstract:The interpretation of X-Ray Absorption spectra in terms of electronic structure has long been of interest. Hole counts derived from such spectra are often interpreted in terms of free-atom occupation numbers or Mulliken counts. It is shown here, however, that renormalized-atom (RA) and cellular counts are better choices to characterize the configuration of occupied electron states in molecules and condensed matter. A projection-operator approach is introduced to subtract delocalized states and to determine quantitatively such hole counts from X-Ray Absorption spectra. The described approach is based on multiple-scattering theory (MST) and on atomic calculations of a smooth transformation relating the X-Ray Absorption spectra to local projected densities of states (LDOS). Theoretical tests for the s and d electrons in transition metals show that the approach works well.
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Multiple-scattering calculations of X-Ray-Absorption spectra
Physical review. B Condensed matter, 1995Co-Authors: S. I. Zabinsky, A. L. Ankudinov, John J. Rehr, Robert C. AlbersAbstract:A high-order multiple-scattering (MS) approach to the calculation of polarized X-Ray-Absorption spectra, which includes both X-Ray-Absorption fine structure and X-Ray-Absorption near-edge structure, is presented. Efficient calculations in arbitrary systems are carried out by using a curved-wave MS path formalism that ignores negligible paths, and has an energy-dependent self-energy and MS Debye-Waller factors. Embedded-atom background Absorption calculations on an absolute energy scale are included. The theory is illustrated for metallic Cu, Cd, and Pt. For these cases the MS eXpansion is found to converge to within typical eXperimental accuracy, both to eXperiment and to full MS theories (e.g., band structure), by using only a few dozen important paths, which are primarily single-scattering, focusing, linear, and triangular.
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Sum rules for polarization-dependent X-Ray Absorption
Physical review. B Condensed matter, 1995Co-Authors: A. L. Ankudinov, John J. RehrAbstract:A complete set of sum rules is obtained for polarization-dependent X-Ray-Absorption fine structure and X-Ray circular magnetic dichroism (CMD), analogous to those for CMD derived by Thole et al. These sum rules relate X-Ray-Absorption coefficients to the ground-state eXpectation values of various operators. Problems with applying these sum rules are discussed.
Heiko Wende - One of the best experts on this subject based on the ideXlab platform.
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Recent advances in X-Ray Absorption spectroscopy
Reports on Progress in Physics, 2004Co-Authors: Heiko WendeAbstract:Using X-Ray Absorption spectroscopy recent progress is achieved all over in solid state physics. This review focuses on these advances, with particular emphasis on applications to surface physics and to magnetism of ultrathin 3d and 5d films that are made possible by the use of undulators in third generation synchrotron radiation sources: the unambiguous appearance of an atomic eXtended X-Ray Absorption fine structure for atomic adsorbates and of σ* resonances in near-edge X-Ray Absorption fine structure spectra of oriented molecules is demonstrated. The induced magnetism at the interfaces of 3d and 5d layers is studied by X-Ray magnetic circular dichroism. Fundamental aspects of the spectroscopy are clarified for rare earth crystals. The determination of the ground state properties and the detailed understanding of the underlying mechanisms was obtained by comparison of the eXperimental data to state-of-the-art ab initio calculations.
Peter Behrens - One of the best experts on this subject based on the ideXlab platform.
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X-Ray Absorption spectroscopy in chemistry: I. EXtended X-Ray Absorption fine structure
TrAC Trends in Analytical Chemistry, 1992Co-Authors: Peter BehrensAbstract:Abstract X-Ray Absorption spectroscopy (XAS) is still a relatively new method in chemistry. X-Ray Absorption spectra eXhibit edges, which contain fine structures designated as XANES (X-Ray Absorption near edge structure) and EXAFS (eXtended X-Ray Absorption fine structure). The information content of these structures is demonstrated by eXamples taken from solid-state, surface and bio-inorganic chemistry. In this first part, the determination of bond distances and coordination numbers as well as the derivation of information on order and disorder from EXAFS data is presented.
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X-Ray Absorption spectroscopy in chemistry. II: X-Ray Absorption near edge structure
TrAC Trends in Analytical Chemistry, 1992Co-Authors: Peter BehrensAbstract:Abstract The edges in X-Ray Absorption spectra eXhibit fine structures designated as XANES (X-Ray Absorption near edge structure) and EXAFS (eXtended X-Ray Absorption fine structure). The information content of EXAFS with regard to interatomic distances, coordination number, and the determination of degrees of order was presented in the first part of this article [Trends Anal. Chem., 11 (1992) 218]. The second part deals with the fine structures near to the edge, the XANES region, which contains information about the electronic and the geometric structure of the absorbing atom.
