EXAFS, principles

Teo BK (1986) EXAFS, principles and data analysis. Springer, Berlin... 

Teo, B. (1986) EXAFS Basic Principles and Data Analysis, Springer, Berlin. 

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. Principles, Applications, Techniques of EXAFS, SEXAFS andXANES. (D. C. Koningsberger and R. Prins, eds.) Wiley, New York, 1988. 

X-Ray Absorption Principles,Applications,Techniques of EXAFS, SEXAFS, and XANES. Edited by D. C. Konnigsberger... 

Such a function exhibits peaks (Fig. 9C) that correspond to interatomic distances but are shifted to smaller values (recall the distance correction mentioned above). This finding was a major breakthrough in the analysis of EXAFS data since it allowed ready visualization. However, because of the shift to shorter distances and the effects of truncation, such an approach is generally not employed for accurate distance determination. This approach, however, allows for the use of Fourier filtering techniques which make possible the isolation of individual coordination shells (the dashed line in Fig. 9C represents a Fourier filtering window that isolates the first coordination shell). After Fourier filtering, the data is back-transformed to k space (Fig. 9D), where it is fitted for amplitude and phase. The basic principle behind the curve-fitting analysis is to employ a parameterized function that will model the... 

Teo, B. K. EXAFS Basic principles and data analysis, Springer-Verlag Berlin, 1986. 

Stohr, J. in X-ray Absorption Principles, Applications Techniques of EXAFS, SEXAFS and XANES (ed. Prins, R., Koninsbergen, D.) Wiley, New York... 

The photoelectron wave-vector k is evaluated using = 2m(E — E ) where E is the energy of the X-ray photon, , a reference energy and m, the mass of the electron. x(k) is multiplied by k"(n = 2 or 3 usually) to magnify the faint EXAFS at large k (Lytle et al, 1975) /c"x(k) is Fourier transformed to yield the RSF, < (R). In the model compound, the first peak at a distance Rj represents the distance to the nearest-neighbour shell and may be compared to R[, the known distance. We can then define a as (R — Rj), which represents the experimentally determined phase correction. In principle, 2a should be equal to the theoretically estimated k-dependent part of /k), viz. if the identity of the scatterer environment has been correctly assumed. It must be emphasized that wherever scatterer identities are obscure (e.g. in several covalently bonded and disordered systems) use of a (and not j) is advisable. Further, the k-dependence of < /k) introduces an intrinsic limitation to its quantitative accuracy. 

The EXAFS spectrum of a metal atom in a protein gives the lengths of the bonds between the metal and its ligand atoms with sufficient accuracy to calculate their bond valences. In principle it can also determine the number of each type of bond that the metal forms, but the results are much less reliable. However, calculation of the bond valences sum for the possible environments allows one to determine not only the coordination number but also the oxidation state of the metal atom (Thorp 1992 Hati and Datta 1995 Scarrow et al. 1996 Bell et al. 1997 Clark-Baldwin et al. 1998 Dooley et al. 1998). 

In principle, EXAFS information may be obtained for most or all of the elements in a catalyst. Thus, for multicomponent samples, the characterization of local surroundings for all (or almost all) the elements may be obtained. However, we stress that the radial distribution function cannot be transformed into a unique three-dimensional structure. Therefore, the EXAFS technique is not ideal for providing such information and the data representing materials consisting of several different phases may often be too difficult to analyze meaningfully. 

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