SURFACE-STRUCTURE DETERMINATION USING X-RAY STANDING WAVES - A SIMPLE VIEW

In the application of x-ray standing wave (XSW) methods to the determination of surface structures, the experiment provides two structural parameters: the coherent position and the coherent fraction. For simple, single-high-symmetry-site adsorption systems, the interpretation of these parameters in terms of a structural model is trivial, but in the case of lower-symmetry adsorption sites, or multiple adsorption sites (including those associated with coincidence lattice structures), these parameters are related to spatial distribution functions through a Fourier integral. A particularly simple way of viewing this result, in terms of vector (Argand) diagrams, allows many simple cases and general theorems concerning the interconnection of the structure and the XSW fitting parameters to be visualized. The application of this approach is illustrated with particular reference to recent studies of adsorption on FCC (111) metal surfaces, but some generalization to other surfaces is included.