Surface Modes of Vibration in the Rigid-Ion Model of NaCl

A discussion is given of the complete spectrum of vibrational surface modes for the Kellermann rigid-ion model of NaCl. The attenuation with distance from the surface is shown for representative surface and pseudosurface modes. We discuss one interesting feature which has not previously been pointed out: Symmetry requirements can lead to surface modes which are associated with only one species of ion (${\mathrm{Na}}^{+}$ or ${\mathrm{Cl}}^{-}$) at the surface.