Measurement of the surface dynamics of RbCl(001) via high resolution He atom scattering

High resolution He atom scattering experiments have been carried out on the cleaved RbCl(001) surface in the 〈100〉 direction. For most of the experiments reported here the incident He atom wave vector ki was ∼8 A−1 (corresponding to incident energy Ei≊33 meV) and the target temperature was ∼190 K. Both the Rayleigh wave and a surface optical mode lying in the gap between the bulk acoustic and optical bands (the S2 mode) were observed. The measured surface dispersion branches are compared with model calculations of surface dispersion for both a relaxed and an unrelaxed RbCl(001) surface. The calculated Rayleigh mode dispersion is very nearly the same in both models and agrees well with the experimental results. The experimental optical mode data appear to fit the calculations only in part for each model. A comparison of the RbCl results with previously measured KBr surface dispersion curves shows that their dynamics are largely the same, but with significant differences for part of the S2 mode close to the...