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

The Ag(110) surface was studied at various stages of annealing and ion bombardment cycles by means of reflection-anisotropy spectroscopy (RAS) and scanning tunneling microscopy (STM). At low fluence and a limited number of heat treatment cycles, a positive RAS signal at 3.8 eV, followed by a negative peak at 3.9 eV, was recorded. STM showed that the room-temperature surface corresponding to this curve profile had steps whose edges were parallel with 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 curves where the positive low-energy component at 3.8 eV is absent. We discuss the spectra in terms of local-field calculations where the screened dipole-dipole interaction coefficients are modified by surface steps. If the step edges are isotropically distributed instead of parallel with the [110] direction, the strength of the low-energy part of the RAS curve is reduced. However, the calculated reduction in strength is not enough to account for the experimental results. Step-induced coupling to surface plasmons is an additional mechanism, which gives a much stronger reduction in strength. The influence of both effects on the RAS curve increases with decreasing correlation length. The plasmon-based mechanism takes place already at lengths of the order of 10(2) nm, whereas the cut-off in the dipole-dipole interaction needs correlation lengths that are almost one magnitude lower to be important. (C) 2000 Elsevier Science B.V. All rights reserved.