HREELS INVESTIGATION OF PHYSISORBED MOLECULES ON SINGLE-CRYSTALLINE SURFACES THROUGH DIPOLE AND RESONANT ELECTRON-SCATTERING MECHANISM

Monolayer (ML) and multilayer of CO and N2, physisorbed on Al(111), Ag(111), and Pd(111) surfaces at about 20 K have been investigated by high-resolution electron energy-loss spectroscopy (HREELS) and UV photoelectron spectroscopy (UPS). In the physisorption state, electron scattering via a negative-ion resonance (NIR) is the dominant process for N2 at all coverages and for CO above the ML. Finger-prints of NIR scattering are a resonance in the primary energy of the excitation probability, the peculiar angular distribution of the scattered electrons, and the high overtone intensity of the intra- and inter-molecular vibrations. The lifetime of the NIR is discussed and examples for complete quenching of the NIR in physisorbed N2 and CO are presented.