ADSORPTION OF N2 ON THE PD(111) SURFACE AT 20-K .2. ARUPS AND HREELS OF THE PHYSISORBED PHASE

The adsorption of N2 on Pd(111) has been investigated by means of angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and high-resolution electron energy-loss spectroscopy (HREELS). For N2, which is adsorbed on top of the N2 ML or on top of a Xe spacer layer, a vibrational fine structure of the 1-pi(u) level is observed, whose analysis reveals the existence of the delocalization of the photo-hole during the photoemission process. The 3-sigma(g) level shows a pronounced dispersion, whereas no band formation seems to occur in the case of the 2-sigma(u) level. The nu(N-N) stretching vibration of physisorbed N2 lies at the gas-phase value of 289 meV. It is dipole-inactive, as it is in the gas phase and is excited through negative ion resonance (NIR) scattering. In agreement with a recent observation, it is found that the line shape of the losses observed through NIR scattering is characterized by an asymmetric tail at the high energy side, which is due to the simultaneous excitation of the nu approximately(substrate-N2) mode.