Measurement of the Dispersion Relation for Rayleigh Surface Phonons of LiF(001) by Inelastic Scattering of He Atoms

Time-of-flight spectra of low-energy (≈20 me V) He scattered from a LiF(001) crystal along the $〈100〉$ direction reveal up to five sharp inelastic-scattering maxima. Their scattering angle and flight time determine the frequencies and wave numbers of surface phonons over the entire Brillouin zone. The results agree well with calculations of Rayleigh modes by de Wette and Benedek except near the zone boundary. Further unidentified features are also observed.