Keywords for Solid State Communications

Abstract

Studies show that the X-ray absorption fine structure techniques near-edge region within some ten eV above the edge (NEXAFS) and structure in the extended region well above the edge (SEXAFS) are complementary and can yield detailed information about atomic and molecular adsorption geometries, including adsorbate–substrate distances, adsorption sites, surface reconstructions, intramolecular bond lengths, and molecular orientations. The X-ray absorption fine structure above the absorption edge of an adsorbateatom core electron exhibits quite different features because of different underlying physical processes. Strong and relatively sharp features in the near-edge region within some ten eV above the edge (NEXAFS) are followed by a less pronounced structure in the extended region well above the edge (SEXAFS) as can be observed for CO adsorbed on Ni(111). In the SEXAFS region single-scattering (backscattering) processes of the emitted photoelectrons dominate, such that adsorbate–substrate bond lengths and adsorption sites can be obtained from a SEXAFS measurement in a straightforward manner independent of theory. The accuracy of SEXAFS-derived bond lengths is roughly ±0.02 Å. More stable and more intense synchrotron radiation facilities of the second generation combined with better monochromators have recently improved the quality of SEXAFS data and allowed new structural problems to be tackled, particularly in the area of low-Z elements. Thus, quite recently adsorbate-induced reconstruction has been investigated with SEXAFS, and it is shown in this chapter that intramolecular bond lengths can be determined by measuring SEXAFS above absorption edges of different atoms constituting the bond.