Electron-energy-loss studies of physisorbedO2andN2on Ag and Cu surfaces

Abstract

High-resolution electron-energy-loss spectroscopy is used to study the adsorption of ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$ 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 ${\mathrm{O}}_{2}$. Values of -1.6\ifmmode\pm\else\textpm\fi{}0.2 meV and 5.8\ifmmode\pm\else\textpm\fi{}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 ${\mathrm{N}}_{2}$ 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 ${\mathrm{N}}_{2}$, 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 $^{1}\ensuremath{\Delta}_{g}$ and ${^{1}\ensuremath{\Sigma}_{g}}^{+}$ electronically excited states of ${\mathrm{O}}_{2}$ and their vibrational progressions are also observed. The origins (O-O bands) of these excited states are red-shifted by only \ensuremath{\sim} 10 and \ensuremath{\sim} 30 meV, respectively, from their gas-phase energies.