Interpretation of the ‘‘sagittal resonance’’ in the surface vibrational spectrum of some alkali halides

We demonstrate that the so-called ``sagittal resonance'' which is found in the surface dynamical spectra of NaF, NaCl, and KCl, and which has been detected for KCl with low-energy He scattering, has the vibrational pattern of a TO surface mode. At the Brillouin-zone boundary this low-lying surface mode resembles a Rayleigh mode, but with the light particle vibrating instead of the heavy one.