Surface Phonon Dispersion

Abstract

The concept of surface phonons has emerged from the lattice dynamics of the semi-infinite periodic solid. Therefore, it addresses a highly idealized situation which, at best, can be approached though never realized experimentally. Substantial and important experimental surface research is, however, directed towards less idealized systems. The study of reactions of gas-phase molecules with surfaces may serve as an example. Vibration spectroscopy has become the tool for investigating the chemical nature of surface species, to determine binding sites, and to study the chemical bonds within the adsorbed species as well as to the surface. The vibrating adsorbed species is thus an isolated entity with no coupling to neighboring molecules. The substrate essentially appears as a medium to support the adsorbates without participating in the vibrational motion. As successful as this concept was, and as it remains in many cases, it is bound to break down when lateral interactions between adsorbed species gain importance, e.g., in dense overlayers, or when adsorbate-substrate vibrations with frequencies comparable to the eigenmodes of the substrate are considered. If the adsorbed layer is disordered, the vibration spectrum will consist of intrinsically broad bands, such as one finds for a solid in an amorphous state. Obviously, this situation is not easily amenable to quantitative experimental and theoretical study. Fortunately, however, nature has provided us with an abundance of ordered overlayers of adsorbed species on single-crystal surfaces. It is there where the concept of surface phonons is brought to bear, and where it is going to provide us with new insights into the lateral coupling between surface species and into the nature of the surface chemical bond in densely packed overlayers. The results will also eventually relate to the geometrical structure and possibly also to the transitions between different surface phases.