Time-resolved two-photon photoemission from metal surfaces

The Rydberg-like series of image-potential states is a prototype system for loosely bound electrons at a metal surface. The electronic structure and the femtosecond dynamics of these states is studied by high-resolution energy-and time-resolved two-photon photoemission spectroscopy. The electron trapped in the image potential moves virtually freely laterally to the surface where it is subject to inelastic and quasielastic scattering processes which cause decay of population and phase relaxation. The influence of surface corrugation on these processes has been investigated for adsorbates on Cu(001) and stepped Cu(117) and Cu(119) surfaces which are vicinal to Cu(001). The dynamics depend on both the distance of the electron in front of the surface and the parallel momentum. For CO molecules on Cu(001) inelastic scattering into bulk states and adsorbate-induced resonances determine the decay rate. For small numbers of Cu adatoms on Cu(001) and the vicinal surfaces the decay rate of image-potential states is significantly modified by interband and subsequent intraband scattering. On the vicinal surfaces the origin of the interband-scattering process is clarified as quasielastic scattering caused by disorder of the lateral superlattice. It occurs for electrons with group velocity perpendicular to the step edge and, moreover, exhibits a sizeable asymmetry. Electrons are mainly scattered into states with momentum in the upstairs direction. This asymmetry in quasielastic scattering explains the direction dependence of the lifetime of the first image-potential states on stepped Cu(119).