LIFETIME OF IMAGE-POTENTIAL STATES ON METAL-SURFACES

High-resolution bichromatic two-photon photoemission spectroscopy was used for an investigation of intrinsic linewidths and, thus, lifetimes of image-potential states on clean and adsorbate-covered metal surfaces. The linewidths of the first image state are 16+/-4 and 70+/-8 meV for Cu(111) and Ni(100), respectively. Together with the results for Ag(100) and Ni(111) published previously, a systematic comparison with theoretical predictions is presented. The discrepancies between experiment and theory can be understood qualitatively if the actual band structure is taken into account. A high density of unoccupied bulk or surface states offers many channels for the decay of the image state via electron-hole pair production and results in the large linewidth observed on the nickel surfaces. The opposite is found for Cu(111) where the band gap extends below E(F). For oxygen adsorption on Ni(100), an increase of the linewidth is observed, which can be explained by the lateral confinement of the image state and inelastic scattering by the adsorbate atoms.