the outgoing photoelectron (43). The near-edge or X-ray Absorption Near-Edge Structure (XANES) region lies approximately 8-40 eV beyond the edge, before the true EXAFS region. In XANES, low-energy photoelectrons are strongly scattered, and multiple scattering is believed to dominate (44). Structural information can be obtained from the near-edge region, but multi ple scattering must be included in cal culations to describe the near-edge structure, making data analysis consid erably more complicated than for nor mal EXAFS. Still, there is much inter est in XANES; ultimately it can pro vide greater structural information than EXAFS because bond angle infor mation is contained in the multiple scattering processes. Data analysis
Figure 7. Expanded X-ray absorption spectrum of CoAI204 indicating the pre-edge, edge, near-edge, and EXAFS regions. 1er factor, defined as the root mean square displacement of the average nearest neighbor distance. The DebyeWaller factor can be separated into components because of thermal (vibra tional) and static disorder (i.e., σ2 = a stat + σ?ϋ>) by studying the tempera ture dependence of the EXAFS ampli tude. The Debye-Waller approxima tion assumes that vibrational motion is harmonic and that static disorder is Gaussian. These assumptions are valid for systems of low disorder (38,39), but in systems with a high degree of static disorder or nonharmonic vibration, the Debye-Waller approximation breaks down and can lead to incorrect struc tural results (40). Under such circum stances, a pair distribution function must be used to approximate structur al disorder; this generally increases the complexity of data analysis. Pre-edge, edge, and near-edge regions Additional information concerning the state and environment of the central absorber is found in the pre-edge, edge, and near-edge regions. Figure 7 shows an expanded view of the K-edge spectrum of C0AI2O4. The small preedge peak, seen just below the edge in Figure 7, is often observed in K-edge spectra and is attributed to a transition of the Is electron to valence levels (i.e., Is -* 3d) (41). Pre-edge transitions are sensitive to the symmetry of the ab sorbing atom, and the intensity of the pre-edge peak is often used as evidence of a particular symmetry for the first coordination shell. The absorption edge commonly con tains "ledges" and a distinctive peak at the crest (called the "white line"). The
ledges in the edge have been attributed to transitions to excited s states, and the white line is associated with transi tions to excited ρ states (41, 42). How ever, theoretical calculations have sug gested that these edge features are bet ter described as "shape resonances" of
Data reduction in EXAFS is a multistep process that involves a number of critical data manipulations. Because of this, a consistent approach to data re duction is a prerequisite. An outline of a common (but not the only) route for EXAFS data reduction is shown in the box (p. 1234 A). The data reduction process is divided into two separate phases: Primary and Secondary Data Reduction. Primary Data Reduction takes the original X-ray absorption spectrum and extracts the single shell
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988 · 1233 A
