Physisorption of N 2 and CO on Al(111): A combined HREELS-UPS investigation

Abstract

Mono- and multilayers of CO and N 2 physisorbed on Al(111) at 20 K were investigated by means of high-resolution electron energy-loss spectroscopy (HREELS) and UV photoelectron spectroscopy (UPS). From the desorption temperature an upper limit for the bonding energy to the Al(111) surface of 0.2 eV is estimated for both CO and N 2. Essentially, UPS and HREELS reproduce the gas phase data. The UP spectra are considerably broadened and shifted by 1 to 2 eV to smaller electronic binding energies. The energies of the molecular stretching vibrations are equal to the gas phase value within ± 1 meV. At the CO monolayer the electrons are inelastically scattered in the dipole mode. From the strong scattering cross section a complete in-plane orientation of CO can be excluded. The strong cross section for N 2 is due to resonance scattering via a known temporary negative-ion state.