MULTIPLE-SCATTERING EFFECTS IN POLARIZATION-DEPENDENT SURFACE EXAFS

We investigate the effect of multiple scattering (MS) on the X-ray absorption spectrum using full curved wave theory with exact polarisation dependence and including orders of MS up to five. As well as the EXAFS, much of the near-edge region can be fitted using an algorithm which includes a sufficiently small number of MS paths to make routine calculation feasible. Results are presented for the model c(2x2) overlayer of atomic O in the fourfold hollow on an unreconstructed Cu(100) surface and in the long bridge site on the (110) surface. MS is important for some polarisation directions, and failure to include at least the dominant double scattering terms may result in errors in the parameters for some non-nearest neighbour distances. We derive general rules for when MS is likely to influence surface EXAFS. The dominant MS paths involve the shortest bonds in the material. Most of the significant paths involve either one small (<45-degrees) or large (>150-degrees) scattering angle, with the most favourable geometry for MS being when forward scattering occurs at a light atom and backscattering at a heavy atom. Polarisation dependence tends to select those contributions where the epsilon-vector is roughly parallel to one of these bonds. Multiple scattering effects are likely to be strongest when adsorbate atoms occupy sites close to coplanar with the surface. The presence of MS peaks at high-R in a spectrum is a useful indication of high symmetry and relatively long range order in surface structures.