Surface phonon modes of the RbBr(001) crystal surface by inelastic He-atom scattering

Inelastic He-atom scattering ($k_i$≊7 A${̊}^{\mathrm{-}1}$) was used to measure the dynamics of the RbBr(001) crystal surface ($T_s$≊115 K). Time-of-flight spectra taken in two high-symmetry directions, 〈100〉 and 〈110〉, were analyzed to give the surface dispersion curves for the entire surface Brillouin zones Γ¯ X¯ and Γ¯ M¯. The Rayleigh mode and its geometric ‘‘folded,’’ optical surface resonance (expected from the quasimonatomic behavior due to the nearly equal masses of the ${\mathrm{Rb}}^{+}$ and ${\mathrm{Br}}^{\mathrm{-}}$ ions) were observed; the data also suggest the existence of another optical surface feature in the optical band. Although calculations had suggested that this crystal should have important surface relaxation effects, the main one being a surface-localized optical branch lying above the optical band, no evidence of this was seen. Moreover, the calculated surface dispersion curves for the unrelaxed surface appear to fit the measurements slightly better than those for the relaxed surface, although neither predicts the optical resonance mentioned above. Finally, these observed differences provide guidance on the use of the shell model for describing the physics of ionic insulator crystals.