He atom scattering study of the surface dynamics of an epitaxial one monolayer KBr film on RbCl(001) : a shell model analysis

Abstract High resolution inelastic He atom scattering experiments have been carried out on an epitaxially grown one monolayer film of KBr on RbCl(001) in the temperature range of 180–220K for the ΓM direction. This adsorbate/substrate combination represents a crystal interface with a misfit less than 0.2%. Measured time-of-flight spectra yield the surface dispersion of the film with branches corresponding to a Rayleigh wave, a longitudinal resonance and a surface optical mode that lies in the gap between the KBr bulk optical and acoustic bands. The Rayleigh mode of the film is very similar to the Rayleigh modes for the cleaved (001) surfaces of both KBr and RbCl; however, the optical mode is softened by about ten percent. Because of the small misfit in lattice constants, the unit cell can be approximated by that of the substrate, RbCl. Thus, this system lends itself to a theoretical treatment by the shell model approach, similar to that which has guided theoretical work on the surface dynamics of bulk ionic crystals. Further, shell model parameters can be taken from the literature for the interactions within the overlayer and within the substrate, and adapted for the interaction between the deposited ions and the substrate ions at the interface. A suitable and consistent procedure for the latter has proven to be the most challenging. The surface dispersion and relaxation are calculated by the slab dynamics technique in the usual way. A comparison between experiment and theory is good on the whole considering that the model is without adjustable parameters. In particular, the calculation predicts the softening for the optical gap mode. The calculation also predicts an outward relaxation of the KBr monolayer which is found by the measurements. However, the extent of the relaxation is not predicted to be as great as was observed.