X-ray absorption near edge structure measurements

A structural study of dibromo(l,4,8,ll-tetraazacyclo-tetradecane)copper(II) and diaqua(l,4,8,ll-tetraaza-cyclotetradecane)copper(II) difluoride four hydrate complexes in crystal and in aqueous solution by X-ray absorption near edge structure measurements and DV-Xa calculations... 

Figure 3. Comparison of the rate of oxidation of Fe(II) when mixed with apoferritin coats (480 Fe/molecule) in 0.15 M Hepes Na, pH 7.0, using absorbance at 420 nm ( a—s— ), availability to react with -phenanthroline ( o—o—o ), change in the x-ray absorption near edge structure (XANES) ( — — ). All three types of measurements were made under the same experimental conditions, including the sample holder. (Data are taken from Ref.

In the first section will be presented XAS from the physical principles to data analysis and measurements. Then section 2 will be devoted to a discussion of a few examples to illustrate the power and limitations of XAS for gaining structural information. Examples are focused on EXAFS studies on nanocrystalline materials. Detailed reviews for applications on other fields of materials science or for presenting the complementary information available by the study of the X-ray Absorption Near Edge Structure (XANES) part of the X-ray absorption spectrum can be found in a number of books [3-5], A brief overview of the recent development of the technique regarding the use of X-ray microbeams available on the third generation light sources will be finally presented in the last section. 

The utility of sulfur K-edge X-ray absorption spectroscopy for the determination and quantification of sulfur forms in nonvolatile hydrocarbons has been investigated. X-ray Absorption Near Edge Structure (XANES) spectra were obtained for a selected group of model compounds, for several petroleum asphaltene samples and for Rasa coal. For the model compounds the sulfur XANES was found to vary widely from compound to compound, and to provide a fingerprint for the form of sulfur involved. The use of third derivatives of the spectra enabled discrimination of mixtures of sulfide and thiophenic model compounds, and allowed approximate quantification of the amount of each component in the mixtures, in the asphaltene samples and the coal. These results represent the first demonstration that nonvolatile sulfide and thiophenic sulfur forms can be distinguished and approximately quantified by direct measurement. 

We have yet to obtain vibrational data for complexes [Cu2(Nn)(02)]2+ (5), but a variety of other evidence is consistent with their peroxo-di-copper(II) formulation. These 02-adducts (5) possess low-energy weak d-d absorptions, diagnostic of Cu(II) and not Cu(I), which has a filled shell d10 electronic configuration. X-ray absorption near-edge structure (XANES) measurements carried out on the N3 and N4 derivatives confirm the Cu(II) oxidation state (34), and extended X-ray absorption fine... 

Minerals are identified by a variety of techniques. Some minerals can be identified on the basis of their elemental compositions, but diffraction or X-ray absorption near-edge structure (XANES) spectroscopy provides more compelling mineral identification. Diffraction patterns, produced either by electron or X-ray beams, provide a direct measurement of the spacing between planes of atoms in the crystal structure. When coupled with element analysis, diffraction provides mineralogical identification. 

Can be undertaken (as formerly) by measuring the intensity of absorption of x-rays by a sample placed between a detector and standard x-ray source now largely superseded by the techniques of Extended x-ray-absorption fine structure (EXAFS) and X-ray-absorption near-edge structure (XANES). See entries for these... 

Solid-phase speciation has been measured both by wet chemical extraction and, for arsenic, by instrumental methods principally X-ray absorption near edge structure spectroscopy (XANES) (Brown et al., 1999). La Force et al. (2000) used XANES and selective extractions to determine the likely speciation of arsenic in a wetland affected by mine wastes. They identified seasonal effects with As(El) and As(V) thought to be associated with carbonates in the summer, iron oxides in the autumn and winter, and silicates in the spring. Extended X-ray absorption fine stmcture spectroscopy (EXAES) has been used to determine the oxidation state of arsenic in arsenic-rich Californian mine wastes (Eoster et al., 1998b). Typical concentrations of arsenic in sods and sediments (arsenic <20 mg kg ) are often too low for EXAFS measurements, but as more powerful photon beams become available, the use of such techniques should increase. 

In the present study, we synthesized dibromo(l,4,8,ll-tetraazacyclotetradecane)copper(II) ([CuBr2(cyclam)]) and diaqua(l,4,8,ll-tetraazacyclotetradecane)copper(II) difluoride four hydrate ([Cu(cyclam)-(H20)2]F2 4H20) complexes and performed single crystal structure analysis and X-ray absorption near-edge structure (XANES) measurements in crystals and in aqueous solution. Furthermore, DV-Xa molecular orbital calculations have been made for models based on these results, and the structures and electronic states of the [Cu(cyclam)] complexes in crystals and in aqueous solution are discussed, in particular, on the axial coordination to Cu(II). 

We report results of first-principles molecular orbital calculations on model clusters of graphite (C24), alkali graphite intercalation compounds (MC48 M = Li, Na, K, Rb, Cs), fullerene (Ceo), and fluorinated fullerenes (CeoFa, x — 18, 36, 48). The calculated partial densities of states (pDOS) are compared with measured x-ray absorption near edge structure (XANES) spectra, x-ray photoelectron spectra (XPS), x-ray emission spectra (XES), and ultraviolet photoelectron spectra (UPS). In the case of graphite and its compounds, the calculated pDOS well reproduce features of the observed XANES and UPS spectra. The calculated pDOS and the observed XPS, UPS and XANES of CeoFx (x = 0, 36, 48) are also in satisfactory accordance. 

Additional insight into the nature of the catalytic site was provided by the results of Fe K-edge X-ray absorption near edge structure (XANES) measurements of FeTMPPCl BP [134]. As shown in Figure 3.79, the material pyrolyzed at 800°C, as a dry powder, displays a very pronounced shoulder at energies very similar to those found for metallic Fe, which suggest that the heat treatment leads at least partially to... 

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