ADSORPTION OF N2 ON THE PD(111) SURFACE AT 20-K .1. CHEMISORPTION AND PHYSISORPTION IN THE MONOLAYER

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). The chemisorption of N2 on Pd(111) stops at a coverage which is dilute enough to allow further physisorption of N2 directly on the metal surface. The HREELS signal from chemisorbed N2 is characterized by two losses at 26 meV and 280 meV corresponding to the nu approximately(Pd-N2) and nu(N-N) stretching modes, which suggest an adsorption geometry of N2 on an on-top site with an upright orientation. For physisorbed N2 the nu(N-N) stretching vibration lies at 289 meV (gas-phase value). Besides the 2-sigma(u), 1-pi(u), and 3-sigma(g) levels a final-state peak occurs in the photoemission spectra: it is due to the unoccupied 1-pi(g) N2 level. Adsorbate-induced surface umklapp processes are observed for the (2 x 2) overstructures NO/Pd(111) and CO/Pd(111) but not for N2/Pd(111).