INFLUENCE OF ADSORBATES ON THE IMAGE STATES OF PD(111)

High-resolution two-photon photoemission spectroscopy was used to inve stigate the energy shift and lifetime broadening of the first image st ate on Pd(111) under adsorption of CO, O-2 and H-2 at room temperature and at 90 K. For all adsorbates the binding energy increases with cov erage, which clearly shows that the image state is sensitive not only to the shift of the vacuum level, but also to the variation of the pot ential in the vicinity of the surface caused by the adsorbate The intr insic linewidth of the image state increases under adsorption and impl ies a reduced lifetime of the electron in the image state. It can be e xplained in terms of elastic scattering of the image state at confinin g adcenters and inelastic scattering by these adcenters into bulk stat es.