Model calculation of differential reflectance from ordered adsorbed overlayers

Theoretical calculations are presented on the differential optical reflectance from an ordered overlayer of adsorbates on a metal surface within a simplified model. The adsorbed atoms are assumed to form a square lattice and to interact weakly with the substrate. Attention is focused on the photon energy range where the primary optical excitation involves interband transitions between the highest occupied and the lowest unoccupied two-dimensional adsorbate bands. Structures are observed in the theoretical results for differential reflectance, and are correlated with singularities of the optical joint density of states for the two-dimensional electronic bands participating in the excitation process. The relevance of these calculations to the analysis of experimental spectra is discussed.