Surface phonon dispersion on Cu(111)

We report the first experimental determination of surface phonon dispersion on Cu(111) using high-resolution electron-energy-loss spectroscopy. The gap mode (S₂) near M{} in the surface Brillouin zone is measured for the first time in addition to extending earlier He scattering measurements by Toennies and collaborators of the Rayleigh and longitudinal resonance modes. Lattice-dynamical and electron-scattering analysis of these results indicate that the lateral surface force constant is softened by approximately 15% in marked contrast to earlier suggestions that a 50--70% reduction occurs on this surface.