Electron-energy-loss studies of physisorbed O2 and N2 on Ag and Cu surfaces

Abstract

High-resolution electron-energy-loss spectroscopy is used to study the adsorption of O2 and N2 on Ag and Cu surfaces at about 20 K. We utilize resonance-electron scattering to observe vibrational overtones and low-lying electronically excited states of these adsorbed species. From the overtones we can determine the anharmonicity and dissociation energy for adsorbed O2. Values of -1.6±0.2 meV and 5.8±0.5 eV, respectively, are found which are close to the free-molecule values of -1.49 meV and 6.44 eV. A similar analysis for N2 is less certain due to possible multiple scattering. However, in both cases the data indicate weakly perturbed, i.e., physisorbed species. In the case of physisorbed N2, we observe a more rapid decay of the overtone intensity with increasing vibrational quantum number than in the gas-phase molecule and associate this with a reduced lifetime of the temporary negative-ion state formed during the resonant-electron scattering process. The Δg1 and g1+ electronically excited states of O2 and their vibrational progressions are also observed. The origins (O-O bands) of these excited states are red-shifted by only ∼ 10 and ∼ 30 meV, respectively, from their gas-phase energies. © 1982 The American Physical Society.