The influence of surface steps on the optical and electronic anisotropy ofAg(110)

The Ag(110) surface was studied at various stages of annealing and ion bomb ardment cycles by means of reflection-anisotropy spectroscopy (RAS) and sca nning tunneling microscopy (STM). At low fluence and a limited number of he at treatment cycles, a positive RAS signal at 3.8 eV, followed by a negativ e peak at 3.9 eV, was recorded. STM showed that the room-temperature surfac e corresponding to this curve profile had steps whose edges were parallel w ith the in-plane [110] direction. At longer sputtering times, with several cycles of annealing, we obtained a statistically isotropic distribution of steps and terraces. Ag(110) surfaces of the latter kind result in RAS curve s where the positive low-energy component at 3.8 eV is absent. We discuss t he spectra in terms of local-field calculations where the screened dipole-d ipole interaction coefficients are modified by surface steps. If the step e dges are isotropically distributed instead of parallel with the [110] direc tion, the strength of the low-energy part of the RAS curve is reduced. Howe ver, the calculated reduction in strength is not enough to account for the experimental results. Step-induced coupling to surface plasmons is an addit ional mechanism, which gives a much stronger reduction in strength. The inf luence of both effects on the RAS curve increases with decreasing correlati on length. The plasmon-based mechanism takes place already at lengths of th e order of 10(2) nm, whereas the cut-off in the dipole-dipole interaction n eeds correlation lengths that are almost one magnitude lower to be importan t. (C) 2000 Elsevier Science B.V. All rights reserved.