THEORY OF SURFACE VIBRATIONS IN EPITAXIAL THIN-FILMS

Recent inelastic helium atom scattering measurements of the phonon dis persion curves of epitaxial thin films (2-30 ML) of four different typ es of systems are analyzed within the framework of a force constant mo del. Whereas Na/Cu(001) shows nearly flat dispersionless modes with fr equencies at the Gamma point that follow the open standing wave sequen ce, the other systems Pb/Cu(111), Ar(Kr)/Ag(111), and KBr/NaCl(001) re veal a more complex behavior. These qualitative differences can be exp lained in terms of a simple linear chain model in which the behavior i s determined by the strength of the force constant coupling of the thi n films to the substrate relative to the force constant within the fil ms. Detailed slab calculations for Na/Cu(001) confirm this model and i ndicate that the observed fiat phonon dispersion curves are related to the strength of the interface interaction and bcc structure of the ep itaxially grown Na thin films.