The lattice vibrations of nearly incommensurate overlayers Ag on Ni(100) and Cu(100)

Abstract

A chemisorbed monolayer may form and N × M overlayer, where N or M (or both) may be large. We formulate an approximate description of the lattice vibrations in such structures that includes coupling between the overlayer vibrations, and the substrate phonons. The analysis is a lattice dynamical analogue of Brillouin-Wigner perturbation theory; the scheme is accurate, and readily employed to generate surface spectral densities. A key feature is the ability to analyze the response characteristics for wave vectors in the surface Brillouin zone of the substrate. This greatly simplifies the task of interpreting the output of the numerical computational schemes.We also discuss experimental data on the surface vibrations of a monolayer of Ag on the Ni(100) and the Cu(100) surface, which are closely approximated by a (2 × 8) and (2 × 10) structure, respectively. We achieve a quantitative fit with a simple model, and discuss the physical origin of the interaction between adsorbate and substrate phonons, in regions of the substrate surface Brillouin zone where the adsorbate vibration frequencies lie well below those of the substrate phonons of the same wave vector.