Interpretation of the ‘‘discrepancy’’ in theM¯-point Rayleigh mode in the surface vibrations of LiF(001)

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Abstract
The measured frequencies of the Rayleigh mode of LiF(001) in the 〈100〉 direction of the surface Brillouin zone (SBZ) show a discrepancy with the results of early shell-model calculations. We show that this discrepancy can be removed by either one of two mechanisms: a significant change in the noncentral component of the nearest-neighbor transverse force constant B at the surface, or a small charge transfer between the anion and cation shells at the surface. Since these mechanisms give different predictions for the optical surface modes in the 〈110〉 direction of the SBZ, a choice between them can only be made on the basis of new experimental data.