X-ray Absorption Spectroscopy XANES and EXAFS

Traditionally, X-ray absorption edge measurements have been used to determine oxidation states of metals in complex materials. The extended X-ray absorption fine structure (EXAFS), on the other hand, provides structural information such as bond distances and coordination numbers even with powdered samples, crystalline or amorphous, the fine structure essentially resulting from short-range order around the absorbing atom. The technique is also useful for studying solid surfaces (SEXAFS). The observation of fine structure beyond the K-absorption edges of materials dates back to 

It was noted earlier that EXAFS is a result of two fundamental processes (a) K- (or L-) absorption of an X-ray photon which is the photoelectric effect and (b) an effective diffraction of the electron so emitted. In the case of an isolated absorbing atom (absorber) one sees only the characteristic rise in absorbance, // (= In /(,//), at the energy corresponding to the edge and an exponential decrease thereafter. When the absorber is surrounded by other atoms, // exhibits undulations sometimes up to 2000 eV beyond the edge. Undulations starting 30 eV beyond the edge constitute the EXAFS. As an example, the EXAFS of a bimetallic catalyst is shown in Fig. 2.12. 

Stern and coworkers have treated the effect of such self-interference on the final state with the inclusion of relevant phase-shifts and have shown that the EXAFS, )f(k), can be written as 

A k) represents the total jth shell scattering amplitude and can be written (Sandstrom Lytle, 1979) 

Nj and Oj in equation (2.4) represent the number of atoms in the jth shell and root-mean-square deviation of the interatomic distances over Rj which results both from static and dynamic (thermal) disordering effects respectively. The scattering amplitude, Fj(k) is given by 

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Heald SM, Tranquada JM (1990) X-ray absorption spectroscopy EXAFS and XANES. In Rossiter BW, Hamilton JF (eds) Physical methods of chemistry, vol V. Determination of structural features of crystalline and amorphous solids, 2nd edn. Wiley, New York, p 189... 

In spite of the fairly long history since the discovery and isolation of these enzymes, little has been known on the structure and the catalytic mechanism. The active center of the enzyme is known to be a high-spin Fe +, that has made limited progress in the study of the structure and mechanism due to the spectroscopic inaccessibility. Recently, studies on magnetic circular dichroism (MCD) [128] and X-ray absorption spectroscopy (EXAFS and XANES) [129-132] have brought about some progress in understanding the structures and mechanisms of these enzymes. 

Interest in X-ray absorption spectroscopy techniques and particularly in EXAFS, XANES and SEXAFS, has increased enormously since synchrotron radiation facilities were developed at Stanford University 1974. X-ray absorption spectroscopy is a powerful tool in determining the local structure near a particular atom. X-ray absorption spectra of a surface atom can be divided in two parts ... 

Why is Ce(IV) so active for DNA hydrolysis What factors differentiate this metal ion from other lanthanide ions and non-lanthanide ions Do the f-oibitals of Ce(IV) take significant roles in the catalysis These questions are critically important for practical applications of this catalysis and also from the viewpoints of pure rare earth chemistry. In order to answer them, core-level photoelectron spectroscopy (Shigekawa et al., 1996), as well as EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge stracture) measurements (Shigekawa et al., 1999), were carried out. The spectroscopic analysis was simplified by using diphenyl phosphate (DPP) in place of DNA, and the EXAFS and the XANES measurements were carried out on the samples frozen in liquid nitrogen. 

One of the new fields of actinide chemistry since the 1990s is an application of X-ray absorption spectroscopy (Teo 1986) especially by use of the synchrotron radiation. XAFS (X-ray absorption fine structure) is a powerful technique for characterization of the local structure of specific elements, even radioactive elements such as actinides, and their electronic states. XAFS contains two fundamental information, EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near-edge structure). O Figure 18.27 shows a fundamental pictorial view of the XAFS process (Koningsberger and Prins 1987). 

Important contributions come from the application of X-ray absorption spectroscopy (EXAFS/XANES). Comparing the XANES spectra of dehydrated TS-1 with those of several model compounds, Behrens et al. concluded that TS-1 contains tetrahedral, square pyramidal and octahedral Ti, in the approximate ratio 10 15 75 [57,58] the same authors did not exclude that at ambient conditions all the Ti atoms assume the octahedral coordination [57]. However, these results are affected by the poor quality of the TS-1 sample used, probably containing the majority of the Ti in extra-framework positions, as indicated by the reported value of the unit cell volume (lower than that of the pure silica parent structure) [57,58]. 

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. 

X-ray absorption spectroscopy combining x-ray absorption near edge fine structure (XANES) and extended x-ray absorption fine structure (EXAFS) was used to extensively characterize Pt on Cabosll catalysts. XANES Is the result of electron transitions to bound states of the absorbing atom and thereby maps the symmetry - selected empty manifold of electron states. It Is sensitive to the electronic configuration of the absorbing atom. When the photoelectron has sufficient kinetic energy to be ejected from the atom It can be backscattered by neighboring atoms. The quantum Interference of the Initial... 

The low Ti content (up to 3 wt % in Ti02) makes the extraction of vibrational, energetic, and geometric features specific to Ti04 moieties a difficult task as the experimental data are dominated by the features of the siliceous matrix. This is the reason why the structure of the local environment around Ti(IV) species inside TS-1 was only definitively assessed more than 10 years after the discovery of the material, when the atomic selectivity of X-ray absorption spectroscopies (both XANES and EXAFS) were used [58-60]. 

In general, several spectroscopic techniques have been applied to the study of NO, removal. X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) are currently used to determine the surface composition of the catalysts, with the aim to identify the cationic active sites, as well as their coordinative environment. 

A primary hydration number of 6 for Fe + in aqueous (or D2O) solution has been indicated by neutron diffraction with isotopic substitution (NDIS), XRD, 16,1017 EXAFS, and for Fe " " by NDIS and EXAFS. Fe—O bond distances in aqueous solution have been determined, since 1984, for Fe(H20)/+ by EXAFS and neutron diffraction, for ternary Fe " "-aqua-anion species by XRD (in sulfate and in chloride media, and in bromide media ), for Fe(H20)g by neutron diffraction, and for ternary Fe -aqua-anion species. The NDIS studies hint at the second solvation shell in D2O solution high energy-resolution incoherent quasi-elastic neutron scattering (IQENS) can give some idea of the half-lives of water-protons in the secondary hydration shell of ions such as Fe aq. This is believed to be less than 5 X I0 s, whereas t>5x10 s for the binding time of protons in the primary hydration shell. X-Ray absorption spectroscopy (XAS—EXAFS and XANES) has been used... 

X-ray absorption spectroscopy (XAS) 1. EXAFS 2. XANES Local structure of surface and adsorbed species X-rays in electrons out... 

We thank Dr. M. Tanaka, Dr. Y, Sakai, and Prof. T. Tominaga for Fe Mtissbauer study, and Dr. N. Kosugi and Prof. H. Kuroda for the discussion on XANES and EXAFS results. X-ray absorption spectroscopy was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 87-131). The part of this work is financially supported by the Grant-in-Aid for Scientific Research(No 6247009) from the Ministry of Science and Culture,... 

Fig. 2. a) Required number of incoming x-ray photons to observe time-resolved EXAFS of transition metal compounds in H20 solution with a signal-to-noise ratio S/N = 1. No ligand or counterion contributions were included (see Fig. 1). Input parameters are /= 10%, %= 1 % (relative to the absorption edge jump of the selected element). The maxima of curves 2) in Fig. 1 for Fe and Ru correspond to the data points for these elements, b) Feasibility range for time-resolved x-ray absorption spectroscopy. The shaded region indicates the required x-ray dose per data point as a function of the fraction of activated species for the calculated EXAFS experiments on transition metal compounds shown in a). Curves (1) to (3) are extrapolated from experimental results (see section 3. for details) of time-resolved XANES.

The isomorphous substitution of Silv with Tiiv has also been studied with X-ray absorption spectroscopy at the Ti K edge (XANES and EXAFS). The first results reported indicated that a large proportion of Tilv in TS-1 resided in an... 

Extended X-ray absorption fine structure (EXAFS) spectrum Part of an X-ray absorption spectrum that is used to identify the coordination of atoms, estimate bond lengths, and determine the adsorption complexes on the surfaces of adsorbents. EXAFS spectra may provide useful information on the speciation (valence state), surface complexes, and the coordination of arsenic on adsorbents (e.g. (Randall, Sherman and Ragnarsdottir, 2001 Ladeira, et al. (2001) Teixeira and Ciminelli (2005) Kober, et al. (2005)) (compare with X-ray absorption spectroscopy (XAS), X-ray absorption near edge structure (XANES) spectra, and X-ray absorption fine structure spectroscopy (XAFS)). 

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