ELECTRON-PHONON COUPLING AT METAL-SURFACES PROBED BY HELIUM ATOM SCATTERING

High resolution time-of-flight spectra of helium atoms scattered from single crystal surfaces provide detailed information on surface phonon dispersion curves. For metal surfaces a number of different modes hav e been observed by both helium atom scattering (HAS) and by electron s cattering (EELS). For many metals for which several modes have been de tected simple Born-von Karman force constant models, although successf ul in describing the bulk phonon dispersion curves, are not able to fi t all the available surface data. Recently it has been demonstrated th at a new lattice dynamical model, which accounts for electron-phonon c oupling by the introduction of pseudocharges to accommodate the electr onic dynamical degrees of freedom is however able to explain all the o bservations for several important prototype systems. These include Cu( 001) and Cu(111), the soft mode seen in W(001), and also all the modes , including optical modes in 1T(3)-TaS2. This new model explains the a nomalously large intensities of longitudinal surface phonon resonances only observed in helium scattering on Cu(001) and Cu(111) by accounti ng for the direct interaction of the He atoms with electron charge den sity deformations modeled by pseudocharges.