UNDERSTANDING ATOMIC X-RAY-ABSORPTION FINE-STRUCTURE IN X-RAY-ABSORPTION SPECTRA

The origin and important parameters determining the intensity of atomi c x-ray absorption fine structure (AXAFS) are described both in chemic al and physical terms. A full mathematical derivation is presented and new criteria are given for removal of the background to extract the t otal chi (EXAFS and AXAFS) from the experimental absorption cross-sect ion. The embedded-atom potential, the interstitial potential and the d istribution of the absorber-atom electron density are all found to be important in determining the AXAFS intensity. Application is made to s pherical Pt metal clusters, where it is shown that the AXAFS intensity of the central atom is much larger than that of the surface atoms. Ho wever, the average AXAFS intensity per platinum atom is found not to d epend significantly on cluster size. On the other hand, variation of t he metal cluster support does considerably change the intensity as wel l as the imaginary part of the AXAFS. Hence, AXAFS can be a very usefu l probe of the effects of metal-support interactions in supported nobl e-metal catalysts.