Understanding atomic x-ray absorption fine structure in x-ray absorption spectra

The origin and important parameters determining the intensity of atomic x-ray absorption fine structure (AXAFS) are described both in chemical and physical terms. A full mathematical derivation is presented and new criteria are given for removal of the background to extract the total (EXAFS and AXAFS) from the experimental absorption cross-section. The embedded-atom potential, the interstitial potential and the distribution of the absorber-atom electron density are all found to be important in determining the AXAFS intensity. Application is made to spherical Pt metal clusters, where it is shown that the AXAFS intensity of the central atom is much larger than that of the surface atoms. However, the average AXAFS intensity per platinum atom is found not to depend significantly on cluster size. On the other hand, variation of the metal cluster support does considerably change the intensity as well as the imaginary part of the AXAFS. Hence, AXAFS can be a very useful probe of the effects of metal-support interactions in supported noble-metal catalysts.