Helium time-of-flight spectroscopy of surface-phonon dispersion curves of the noble metals

Abstract

High-resolution inelastic scattering of low-energy (8–17 meV) helium atoms from the (111) surfaces of the three noble metals, Cu, Ag and Au, has been used to determine the surface-phonon dispersion curves for 〈112〉 and 〈110〉 azimuths out to the zone boundary. The new measurements for Cu and Au reveal two sharp dispersion curves corresponding to vibrational modes with a high spectral density at the surface, as observed previously for Ag. The lower-frequency curve is attributed to the Rayleigh mode and the higher-frequency curve to a longitudinal mode, which appears at the surface because of a significant 50–70% weakening in the lateral force constants in the uppermost surface layer. The results are compared with several recent lattice-dynamical calculations.