Anomalous Surface Phonon Dispersion Relations for Ag(111) Measured by Inelastic Scattering of He Atoms

High-resolution time-of-flight measurements of 17-meV He atoms scattered from an Ag(111) surface along the $〈112〉$ and $〈110〉$ azimuths are reported. Most of the spectra reveal two sharp peaks and from their locations dispersion curves have been determined out to the zone boundary. The agreement of the lower-frequency mode with the theoretical Rayleigh dispersion curve is excellent. The other mode is significantly lower (35%) than the calculated longitudinally polarized surface mode.