RESONANCE AND DIPOLE ELECTRON-SCATTERING IN PHYSISORBED MONOLAYERS AND MULTILAYERS ON AL(111) AND AG(111) SURFACES

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.