Proceedings PaperSurface Structure Determinations From Near Edge X-Ray Absorption Fine Structure Studies
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We discuss the x-ray absorption near edge structure (XANES) above the K-edges of light elements in atomic and molecular adsorption on single-crystal surfaces. We concentrate on the systems 0 on Ni(100) and CO on Ni(100), and compare experimental data with the results of multiple-scattering calculations. The r6le of multiple-scattering and the use of lesser approximations is briefly discussed. The atomic adsorption of oxygen provides a reasonably sensitive test of adsorption site, but the near edge structure is influenced by as many as 30 neighbour atoms. By contrast, for the molecular adsorption of CO, the XANES is dominated by intra-molecular scattering with negligible contributions from the substrate atoms. Monitoring the XANES for molecular adsorbates is shown to provide an extremely accurate measure of changes in intra-molecular distances upon chemisorption: an accuracy of 0.01 A is readily achieved, far better than the results of any other surface structural method, and sufficient for the deduction of chemical trends in bonding.