PHYSISORPTION OF CO ON AG(111) - INVESTIGATION OF THE MONOLAYER AND THE MULTILAYER THROUGH HREELS, ARUPS, AND TDS

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 nu-(C-O) stretching mode observed in HREELS and from the application of the dipole selection rule in ARUPS we conclude that the molecules are absorbed 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 5-sigma level exhibits a split which amounts to 0.65 eV at the GAMMA-BAR point and vanishes at k-parallel-to = 0.8 angstrom-1 indicating the formation of an ordered overstructure with more than one molecule in the unit cell. No split at all could be resolved for the 4-sigma 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 2-pi level of CO.