MULTIPLE-SCATTERING EFFECTS IN POLARIZATION-DEPENDENT SURFACE XAFS

The effect of multiple scattering (MS) on the x-ray absorption fine structure (XAFS) spectrum is investigated by means of calculations using the full curved wave theory with exact polarization dependence and including all orders of MS up to five. We performed model calculations for the hypothetical half-monolayer c (2 X 2) overlayer of atomic 0 on an unreconstructed Cu(100) or (110) surface and assuming adsorption in hollow, bridge or atop sites at an O-Cu distance of 1.85 angstrom. From these calculations it is found that MS is important for some adsorption geometries and polarization directions. Failure to include at least the dominant double scattering terms in an analysis may result in errors in the parameters for some distances beyond the nearest neighbors. We derive general rules for when MS is likely to influence surface EXAFS (extended x-ray absorption fine structure) spectra. Most of the significant three-atom paths involve either one small ( < 45-degrees) or large ( > 150-degrees) scattering angle, with the most favorable geometry for MS being when forward scattering occurs at a light atom and backscattering at a heavy atom. The dominant MS paths involve the shortest bonds in the material. Contributions tend to be largest where the epsilon vector is roughly parallel to one of these bonds, and there is often a substantial polarization dependence to the MS terms. 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 an experimental spectrum is a useful indication of high symmetry and relatively long-range order in surface structures. We also show that much of the near-edge region can be fitted using an EXAFS algorithm which includes a sufficiently small number of multiple-scattering paths to make routine calculation feasible, provided that the exact curved wave polarization dependence is included.