Screening effects in photoemission from weakly bound adsorbates: CO on Ag(110)

Abstract

Abstract Photoelectron spectra of the core and valence region of CO adsorbed on Ag(110) at low temperature are reported. The valence spectra show that the large bonding shift of the 5σ level observed for CO adsorbed on transition metals, as well as the large relaxation shifts are absent in CO/Ag(110). This proves that the CO molecule is physisorbed on Ag(110). Polarisation-dependent spectra indicate that CO is not bound to the surface in an upright geometry. The molecule is probably bound to the surface with its molecular axis parallel to the surface, or in a random orientation. The C 1s and O 1s core levels exhibit pronounced satellite structure. By comparison with the spectra from the CO/Ni and CO/Cu adsorption systems, it is shown that the satellite structure and the differences in extra-atomic relaxation shifts can be understood from a unified point of view, by invoking a model in which both core and valence photoelectron spectra are shaped by the strength of the screening of the photon-induced hole through metal-adsorbate charge transfer.