ANGULAR-DEPENDENCE OF LOSS INTENSITIES IN INELASTIC ELECTRON-SCATTERING FOR PHYSISORBED CO ON AG(110)

The a3PI and A1PI excitations of a physisorbed disordered bilayer of C O on Ag(110) have been investigated by means of electron energy loss s pectroscopy (EELS) as a function of electron incidence angle and scatt ering angle and primary electron energy. Whereas in the (sub)monolayer regime these electronic excitations are quenched because of adsorbate -substrate interactions, strong losses with a clearly resolved vibrati onal splitting are observed at coverages above a monolayer. The intens ities of these excitations behave differently as a function of the exp erimental geometry, which can be explained by the different characters of the excitations, i.e. one is optically allowed whereas the other i s optically forbidden. The role the electron trajectories play for the ratio of the cross-sections, of these two excitations is discussed, i ncluding electron-image charge interaction and diffraction effects.