EELS STUDIES OF SURFACE PHONONS ON AG(111)

High-resolution electron energy loss spectroscopy is used to study sur face phonons in the direction of <(Gamma)over bar>(M) over bar on Ag(1 11). The gap mode (S-2) at (M) over bar in the surface Brillouin zone is measured for the first time. We have also measured the Rayleigh mod e and the resonance mode up to the zone boundary. The results are comp lementary to the earlier He atom scattering measurements and are in go od agreement with the first-principles calculations performed recently .