He scattering from disordered surfaces: specular intensity, cross-section and surface structure

Since the specular intensity and scattering matrix are related by a quadratical relation, a term is present in the intensity which scales as the square of the defect surface coverage. One can directly account for the term in every calculation which provides the scattering matrix. The explicit formulas under the sudden approximation are reported and their implications discussed. Any relation between cross-section and specular intensity which does not take it into account is demonstrated to be inadequate even at very low coverage values. Consequently cross-sections calculated on phenomenological grounds cannot give correct information on surface structure.