Scale factor, EXAFS

EXAFS Data Analysis. A key aspect of the analysis outlined above is knowledge of the correct (k) and for a particular absorber-scatterer interaction. These parameters can either be calculated ab initio (6) or can be determined by measuring the EXAFS of structurally characterized model compounds (7). The ab initio method has the advantage that one need not prepare appropriate models for all possible unknowns. Unfortunately however, the ab initio parameters must be adjusted by a scaling factor and an assignment of E, (8). For this reason, one typically calibrates the calculated... 

The quantity D is a scaling factor related to coordination numbers, and Adifference between the value of cr2 characteristic of the platinum L m EXAFS of the platinum-iridium catalyst and the corresponding value of cr2 for a platinum reference material (48). The reference material is pure metallic platinum or a platinum catalyst. The quantity A (K) is the amplitude function of the platinum reference material. It is emphasized that the parameter A different from the analogous parameter in Eq. 4.14. Also, the values of the interatomic distance R in Eqs. 4.14 and 4.17 will not in general be equivalent. 

Photoionization and therefore EXAFS takes place on a time scale that is much shorter than that of atomic motions so the experiment samples an average configuration of the neighbors around the absorber. Therefore, we need to consider the effects of thermal vibration and static disorder, both of which will have the effect of reducing the EXAFS amplitude. These effects are considered in the so-called Debye-Waller factor which is included as... 

In the present study we have extracted the EXAFS from the experimentally recorded X-ray absorption spectra following the method described in detail in Ref. (l , 20). In this procedure, a value for the energy threshold of the absorption edge is chosen to convert the energy scale into k-space. Then a smooth background described by a set of cubic splines is subtracted from the EXAFS in order to separate the non-osciHatory part in ln(l /i) and, finally, the EXAFS is multiplied by a factor k and divided by a function characteristic of the atomic absorption cross section (20). 

Figure 11. X-ray diffraction patterns and EXAFS radial structure functions of two poorly-ciystallized compounds. Both ferrihydrite and silicate glass look amorphous through XRD, but the former is as well short-range ordered as feroxyhite, whereas the latter is disordered even at the local scale. Ge02 is used as the EXAFS example instead of Si02 to better match the second-neighbor scattering factors for the Fe oxides.

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