RAS studies of laterally nanostructured surfaces

With Pt(101) and Ag(110) as examples we discuss the application of Bergman theory to describe optical anisotropy of laterally nanostructured metal sur faces. In both cases the surfaces are characterized by scanning tunneling m icroscopy. The clean Pt(100) surface shows a pseudo-hexagonal reconstructio n. The incommensurability of the top layer with the underlying cubic struct ure leads to a corrugated surface. The observed reflection-anisotropy spect rum (RAS) can be accounted for by a simple choice for the surface spectral density function. In the Ag(110) case we measure and discuss the spectra wh en the surface step edges are parallel with the in-plane [110] direction, a nd when there is a statistically isotropic distribution of steps and terrac es. The spectra are interpreted in terms of local-field calculations where the screened dipole-dipole interaction coefficients are modified by surface steps. Step-induced coupling to surface plasmons is proposed as an additio nal mechanism. The influence of both effects on the experimental reflection -anisotropy curve increases with decreasing correlation length. The plasmon -based mechanism takes place already at lengths of order 10(2) nm, whereas the cut-off in the dipole-dipole interaction needs correlation lengths that are almost one magnitude lower to be important.