EXAFS structure measurements

In the following, structural data are obtained for Ft atoms and their near neighbors on active catalysts under controlled conditions. XANES Is used to Indicate the direction and amount of d-electron flow between the Ft catalyst and Its ligands, EXAFS to measure near neighbor structural parameters. We find EXAFS/XANES to be a sensitive and subtle Indicator of small changes In the environment of catalytic atoms. 

Extended X-ray Absorption Fine Structure (EXAFS) the measurements were mostly made at the Gilda Italian Beamline (equipped with a bending magnet) at the European Synchrotron Radiation Facility in Grenoble (France). 

EXAFS (Extended X-ray Absorption Fine Structure) measurements using synchrotron radiation have been successfully applied to the determination of structural details of SCO systems and have been particularly useful when it has not been possible to obtain suitable crystals for X-ray diffraction studies. Perhaps the most significant application has been in elucidating important aspects of the structure of the iron(II) SCO linear polymers derived from 1,2,4-triazoles [56]. EXAFS has also been applied to probe the dimensions of LIESST-generated metastable high spin states [57]. It has even been used to generate a spin transition curve from multi-temperature measurements [58]. 

Extended x-ray absorption fine structure measurements (EXAFS) have been performed to investigate the short-range structure of TbFe films (46). It is observed that there is an excess number of Fe—Fe and Tb—Tb pairs in the plane of the amorphous film and an excess number of Tb—Fe pairs perpendicular to film. The increase of K with the substrate temperature for samples prepared by evaporation is explained by a rearrangement of local absorbed atom configurations during the growth of the film (surface-induced texturing) (47). 

Furthermore, quantitative structural phase analysis, for instance, is important for investigations of solid catalysts, because one frequently has to deal with more than one phase in the active or precursor state of the catalyst. Principal component analysis (PCA) permits a quantitative determination of the number of primary components in a set of experimental XANES or EXAFS spectra. Primary components are those that are sufficient to reconstruct each experimental spectrum by suitable linear combination. Secondary components are those that contain only the noise. The objective of a PCA of a set of experimental spectra is to determine how many "components" (i.e., reference spectra) are required to reconstruct the spectra within the experimental error. Provided that, first, the number of "references" and, second, potential references have been identified, a linear combination fit can be attempted to quantify the amount of each reference in each experimental spectrum. If a PCA is performed prior to XANES data fitting, no assumptions have to be made as to the number of references and the type of reference compounds used, and the fits can be performed with considerably less ambiguity than otherwise. Details of PCA are available in the literature (Malinowski and Flowery, 1980 Ressler et al., 2000). Recently, this approach has been successfully extended to the analysis of EXAFS data measured for mixtures containing various phases (Frenkel et al., 2002). 

Pt Liii edge EXAFS spectra measured on each isomer confirmed their structures... 

In this work we start with the primitive jellium model, as appropriate for alkaline metals. In the jellium model for metal clusters a fundamental input is the size-dependent ionic density. Fortunately, when one of us started this calculation in 1984 [3], some experimental data about the size dependence of the nearest-neighbor distance were available from EXAFS (extended X-ray absorption fine structure) measurements [19]. Except for fine details the size dependence is very weak. This means that in a first approximation the bulk density of the metal can be used as input for a cluster calculation. A second question is the size dependence of the shape. Since electron micrographs very often show a spherical shape, at least for the larger clusters, a spherical shape will be assumed for all cluster sizes. This means that for monovalent systems the radius R of the jellium cluster is determined by its bulk density... 

The sol-gel method provides uniform fine powders at lower heating temperatures and for shorter heating times than a conventional heating method of mixed oxides. The crystalline sizes of the powders are about 30-50 nm in diameter. X-ray diffraction (XRD) analysis of the prepared YIG and YAG solid solutions revealed garnet structure of A3BSO12 [II] even in the case of Y (AI, Fe) 0 = 4 4 12 starting ratio. In order to investigate where the excess Y atoms are located at, extended X-ray absorption fine structures (EXAFS) were measured. 

We described the detail of the X-ray pinpoint structural measurement system for investigation of optical recording process. Furthermore, we showed the recent progress for fuUy imderstanding the atomic structure of AIST and compare it to GST with the research combined X-ray diffraction, EXAFS, HXPS measurements and computer simulations. These demonstrated that the time resolved X-ray diffraction technique using SR is very pwwerful for the structural investigation of crystal growth phenomena. 

Evidence supporting the presence of several zinc-stabilized polypeptide loops in TFIIIA came from the results of limited proteolytic digestion (Miller et al. 1985). This was corroborated by EXAFS (extended X-ray absorption fine structure) measurements on 7S RNP particles (Diakun et al. 

Various experimental probes on the hydration-shell structure of Cm (aq) reported in literature have yielded a wide range of coordination numbers. To mention a few, EXAFS experiments measured primary hydration numbers of 9 or 7 (based on the truncation of the EXAFS fitting data) in 1M HCIO4 acid [114] and 10 in 0.25 M HCl acid [115]. High energy X-ray scattering (HEXS) experiments yielded a hydration number of 8.8 [114]. Time-resolved laser fluorescence spectroscopy (TREES) found coordination numbers between... 

X-ray absorption spectra for the samples were measured at the Photon Factory in the Institute of Materials Structure Science for High Energy Accelerator Research Organization, Tsukuba, Japan, with a ring current of 2.5 GeV and a stored current of 250-450 mA. Pt Lm-edge EXAFS was measured at the beam hne BL-7C and 9C... 

While X-ray diffraction is well suited to the study of the structure of crystalline samples, it is less well suited to amorphous or disordered ones. In this area, the availability of synchrotron radiation has again led to the development of new techniques, in particular, extended X-ray absorption fine structure (EXAFS). In EXAFS, structural information is derived from an analysis of the oscillations that appear above an X-ray absorption edge, and this technique is now widely applied. The conventional EXAFS experiment is not surface sensitive but some variants are. Under HV conditions, the electron yield is a measure of the absorption, and since these electrons can only escape from a thin layer at the surface, this detection method is surface sensitive. This is the basis of SEXAFS. " Clearly, this cannot be used for in situ studies, but in these cases the fluorescent X-ray yield (which is also a measure of the absorption) can be measured and, particularly when this is coupled with a glancing incidence geometry, reasonable surface sensitivity is achieved. Some initial studies of this type on electrochemical systems have now been made, and these will be described in greater detail later. 

The discovery of the phenomenon that is now known as extended X-ray absorption fine structure (EXAFS) was made in the 1920s, however, it wasn t until the 1970s that two developments set the foundation for the theory and practice of EXAFS measurements. The first was the demonstration of mathematical algorithms for the analysis of EXAFS data. The second was the advent of intense synchrotron radiation of X-ray wavelengths that immensely facilitated the acquisition of these data. During the past two decades, the use of EXAFS has become firmly established as a practical and powerfiil analytical capability for structure determination. ... 

The last three detection schemes apply only under very special circumstances. Transmission EXAFS is strictly a probe of bulk structure, i.e., more than about a thousand monolayers. The electron- and ion-yield detection methods, which are used in reflection rather than transmission schemes, provide surface sensitivity, 1-1,000 A, and are inherendy insensitive to bulk structure. X-ray fluorescence EXAFS has the widest range of sensitivity—from monolayer to bulk levels. The combination of electron or ion yield and transmission EXAFS measurements can provide structural information about the X-ray absorbing element at the surface and in the bulk, respectively, of a sample. 

Surface atomic structure. The integrated intensity of several diffracted beams is measured as a fimction of electron beam energy for different angles of incidence. The measurements are fitted with a model calculation that includes multiple scattering. The atomic coordinates of the surfiice atoms are extracted. (See also the article on EXAFS.)... 

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