XANES


NEXAFS (also called XANES) yes 11 1.0  [c.1754]

Associa.tlons. Organic titanates tend to associate (12). Although a titanium (IV) atom strives to achieve a coordination number of 6 by sharing electron pairs from nearby ester molecules, this tendency may be opposed by stearic crowding. X-ray diffraction (xrd) experiments performed on tetramethyl and tetraethyl titanate single crystals show that, ia the soHd state, these are tetramers (13). In benzene solution, however, cryoscopic measurements suggest that tetraethyl and tetra- -butyl titanate are trimeric tetraisopropyl titanate is monomeric ia nature (14). Titanium nmr experiments support a monomeric stmcture for tetraisopropyl and tetra-/-butyl titanate [3087-39-6] (15). These conclusions are supported by more recent x-ray absorption studies (xanes-exafs), which iadicate only one kind of Ti—O bond distance (1.8 nm) for tetra-/-amyl and tetraisopropyl titanate, whereas there are bond distances of 18.0 nm and 20.5 nm for tetraethyl and tetra- -butyl titanate. These latter have been attributed to terminal and bridging groups on trimeric oligomers, ia which each titanium atom is five coordinate (Fig. 1).  [c.139]

EXAFS is part of the field of X-ray absorption spectroscopy (XAS), in which a number of acronyms abound. An X-ray absorption spectrum contains EXAFS data as well as the X-ray absorption near-edge structure, XANES (alternatively called the near-edge X-ray absorption fine structure, NEXAFS). The combination of XANES (NEXAFS) and EXAFS is commonly referred to as X-ray absorption fine structure, or XAFS. In applications of EXAFS to surface science, the acronym SEXAFS, for surface-EXAFS, is used. The principles and analysis of EXAFS and SEXAFS are the same. See the article following this one for a discussion of SEXAFS and NEXAFS.  [c.215]

XANES X-Ray Absorption Near-Edge Structure  [c.766]

The technique of extended X-ray absorption fine structure (EXAFS) [4.132, 4.133] is based on analysis of variations in the absorption coefficient of a material observed in a range of several hundred eV above the absorption edge of one of the atomic components of the material. Because the incident energy must be scanned in an EXAFS experiment, high intensity over a wide energy range is required and EXAFS is normally applied only at synchrotron radiation sources, where it is now a commonplace technique. When an inner shell electron is ejected from an atom the outgoing photoelectron has a wavelength that depends on its energy. Scattering from surrounding atoms occurs and effectively leads to constructive or destructive interference effects that modulate the measured absorption coefficient. Analysis of these modulations involves background subtraction and then analysis with an appropriate computer program to extract information about the distance and type of the surrounding atoms. EXAFS is a very powerful technique for local structural analysis around particular atomic species of a sample. It is often combined with analysis of the near-edge region, this being called X-ray absorption near-edge structure (XANES) or near-edge X-ray absorption fine structure (NEXAFS), from which information is obtained on the oxidation state and the symmetry of the local environment of the atomic species under investigation.  [c.213]

XANES, and in a-C H films the sp sp ratio can be measured directly using nmr spectroscopy [71].  [c.15]

XANES X-ray adsorption nearedge spectroscopy [178a] Same as NEXAFS Same as NEXAFS  [c.316]

Soon after the development of EXAFS it was recognized that the signal near the x-ray absorption edge is quite complex and provides infonnation on electronic transitions from atomic core levels to valence bands and/or molecular orbitals [69]. The analysis of that signal constitutes the basis for a technique named near-edge x-ray absorption fine stnicture (NEXAFS, or XANES). The shape of the x-ray absorption spectra near the absorption edge has long been used as an empirical fingerprint for the local chemical enviromnent of oxides and other supported catalysts, but newer developments allow for the extraction of a more detailed picture of the nature and geometrical arrangement of adsorbates from those data. This is possible thanks in great part to the combination of the polarized nature of synclnotron radiation and the simplicity of the electronic transition dipoles for absorption from core levels [70]. Fignre Bl.22.10 displays an example where the geometry of vinyl moieties adsorbed on Ni(lOO) surfaces was detennined by using NEXAFS [71].  [c.1792]

Kongingsberger D C and Prins R (ed) 988 X-Ray Absorption Principies, Appiications, Techniques of EXAFS, SEXAFS and XANES (New York Wiley)  [c.1798]

The uranium content of a sample can be determined by fluorimetry, a-spectrometry, neutron activation analysis, x-ray microanalysis with a scanning-transmission electron (sem) microscope, mass spectrometry, and by cathodic stripping voltammetry (8). In most cases, measurements of environmental or biological materials requke preliminary sample preparations such as ashing and dissolution ki acid, followed by either solvent extraction or ion exchange. For uranium isotope analysis, kiductively coupled plasma—mass spectrometry may also be used (81). Another uranium detection technique that has become very popular within the last few years is x-ray absorption near edge stmcture (xanes) spectroscopy. This method can provide information about the oxidation state or local stmcture of uranium ki solution or ki the soHd state. The approach has recently been used to show that U(VI) was reduced to U(IV) by bacteria ki uranium wastes (82), to determine the uranium speciation ki soils from former U.S. DOE uranium processkig faciHties (83,84), and the mode of U(VI) binding to montmorillonite clays (85,86).  [c.323]

Figure 2 Molybdenum K-edge X-ray absorption spectrum, ln(i /i ) versus X-ray energy (eV), for molybdenum metal foil (25- jjn thick), obtained by transmission at 77 K with synchrotron radiation. The energy-dependent constructive and destructive interference of outgoing and backscattered photoelectrons at molybdenum produces the EXAFS peaks and valleys, respectively. The preedge and edge structures marked here are known together as X-ray absorption near edge structure, XANES and EXAFS are provided in a new compilation of literature entitled X-rsy Absorption Fine Structure (S.S. Hasain, ed.) Ellis Norwood, New York, 1991. Figure 2 Molybdenum K-edge X-ray absorption spectrum, ln(i /i ) versus X-ray energy (eV), for molybdenum metal foil (25- jjn thick), obtained by transmission at 77 K with synchrotron radiation. The energy-dependent constructive and destructive interference of outgoing and backscattered photoelectrons at molybdenum produces the EXAFS peaks and valleys, respectively. The preedge and edge structures marked here are known together as X-ray absorption near edge structure, XANES and EXAFS are provided in a new compilation of literature entitled X-rsy Absorption Fine Structure (S.S. Hasain, ed.) Ellis Norwood, New York, 1991.
HgTe crystallizes in the zincblende structure at ambient pressure, transforming to a cinnabar structure at 1.5 GPa, at 300K, and then to a rocksalt structure at 8 GPa. Simultaneously to pressure-induced structural changes, transitions from semi-metal -> semiconductor -> metal occur. The zincblende -> cinnabar -> rocksalt transition leads to important changes in the shape of the XANES spectra at the Hg L3 edge. The goal of these Full Multiple Scattering (FMS) calculations based on a real-space cluster method is threefold we first want to reproduce the XANES modifications observed under increasing pressure, then confirm the atomic positions determined elsewhere by angular dispersive X-ray diffraction (ADX) and XAS by using these data for the construction of the cluster and finally address XANES resonances as fingerprints of the different structures.  [c.447]

The main modifications in the HgTe XANES spectra under pressure are well reproduced only by changing the symmetry around Hg. The good reproducibility proves the accuracy of the atomic positions determined elsewhere. Then the XANES becomes a good test to validate different structural models. This interesting "testing" property is used to determine the phase of HgTe above 12 GPa for which some controversies exist always . Finally, some XANES resonances have been addressed as a fingerprint of the structures. This can be helpful to study phase transitions in mercury chalcogenures without translationnal symmetry like liquids.  [c.450]

Figure 4. Contributions to XMCD. a. thin line C ( thick line C broken line. Left panel XANES right panel EXAFS Figure 4. Contributions to XMCD. a. thin line C ( thick line C broken line. Left panel XANES right panel EXAFS

See pages that mention the term XANES : [c.218]    [c.767]    [c.975]    [c.36]    [c.318]    [c.321]    [c.321]    [c.372]   
Physical chemistry of surfaces (0) -- [ c.316 ]

Encyclopedia of materials characterization (1992) -- [ c.215 ]