Physisorption of CO on Ag(111): investigation of the monolayer and the multilayer through HREELS, ARUPS, and TDS

Abstract

The interaction of CO with Ag(111) has been investigated at temperatures below 35 K through high-resolution electron energy-loss spectroscopy (HREELS), angle-resolved UV photoelectron spectroscopy (ARUPS), and thermal desorption spectroscopy (TDS). From the dipole activity of the v(C-O) stretching mode observed in HREELS and from the application of the dipole selection rule in ARUPS we conclude that the molecules are adsorbed with a random orientation of the molecular axis in the monolayer. No dispersion of the valence levels is observed in the monolayer. In the multilayer the 5σ level exhibits a split which amounts to 0.65 eV at the \\ ̄ gG point and vanishes at k ∥ = 0.8 Å −1 indicating the formation of an ordered overstructure with more tha one molecule in the unit cell. No split at all could be resolved for the 4σ level. The negative ion resonance scattering, known from HREELS measurements in the gas phase, is observed only when the coverage of one monolayer is exceeded, indicating that even in the case of physisorption the metal surface influences the unoccupied 2π level of CO.