Item

Abstract

High-resolution electron energy-loss spectroscopy (HREELS) has been used with UV photoelectron spectroscopy (UPS) and thermal desorption spectroscopy (TDS) to investigate physisorbed N₂ and CO on Al(111) and on Ag(111) with and without H₂O coadsorption. Electron scattering occurs for N₂ at all coverages via a negative-ion resonance (NIR). This can be recognized from the appearance of the characteristic features of this scattering mechanism: (a) the resonant dependence of the cross section on the primary energy, (b) the peculiar angular distribution of the scattered electrons, (c) the strong enhancement for the excitation probability of dipole-forbidden overtones and (d) the asymmetric lineshape of the stretching features due to the excitation of molecule against surface vibrations. No dependence of the scattering mechanism on the layer thickness can be established for N₂, whereas for CO the critical coverage of one monolayer (ML) must be exceeded in order that the NIR scattering mechanism becomes active. The coadsorption of submonolayer quantities of H₂O with a ML N₂ on Al(111) leads to the formation of a mixed phase in which the H₂O molecules are embedded in the N₂ matrix and the NIR is quenched.