The EXAFS Experiment—Data Analysis

The third spectral region is the EXAFS, where the core electron goes into the continuum. As the selected atom absorbs X-rays, an inner shell electron is ejected, producing an outgoing photoelectron wave. This wave is backscattered by the surrounding shell(s) of atoms to yield a new wave with an energy-dependent phase difference with respect to the out-going wave (Fig. 2). This results in either an increase or a decrease of the absorption coefficient of the 

Fourier transformation of the EXAFS data permits it to be replotted as a function of distance (Fig. 9). This aids visualization of the data, since each peak in the Fourier transform in principle represents a shell of atoms. However, because a phase shift of about 0.4 A [1] appears in the Fourier-transformed data, it is not possible to use transformed data to accurately determine M-L distances. Nonetheless, it is possible to use the transformed data to make an initial guess as to the radial distribution function of atoms surrounding the metal. The Fourier transform can also be used to check the background subtraction procedure and the noise level of the spectrum. The presence of peaks of significant intensity at very low R (A) suggests that errors were made in the subtraction process peaks at high R result from especially noisy data. 

Two basic methods of EXAFS data analysis have been developed, one based primarily on theoretical principles [9] and the other based on transferable phase and amplitude parameters derived from the study of structurally defined model compounds [1, 13]. Both methods have been successfully applied to the 

Study of metalloproteins and to heme systems. Most often, the different methods of data analysis produce very similar results. Thus, when disagreements do occur, they can typically be ascribed to differences in the samples or in the measurement of the data, and not to the method of data analysis [2]. 

The theoretically based method [9] suffers from the use of a large number of adjustable parameters in the data analysis. However, an alternate theoretical model has recently been proposed which addresses some of the deficiencies [104]. Specifically, the multiple-scattering method incorporates interactions that occur between the absorbing metal and groups of backscattering atoms, enabling simulation of unfiltered experimental EXAFS data [103-105]. 

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