Selective resonance population in electron scattering by adsorbed molecules.

Abstract

We have investigated vibrational excitation of physisorbed ${\mathrm{O}}_{2}$ by electron impact in the 4--14-eV range, where a single broad resonance is observed in gas-phase studies. We show that by orienting the molecule in two different alignments on the surface of graphite it is possible to selectively populate two different negative-ion resonance states in this energy range, the $^{4}\mathrm{\ensuremath{\Sigma}}_{\mathit{u}}$ state (at \ensuremath{\sim}8.5 eV) and the $^{2}\mathrm{\ensuremath{\Pi}}_{\mathit{u}}$ state (at \ensuremath{\sim}6 eV), each of which leads to molecular vibrational excitation. Calculations show that this selective resonance population arises from the distinctive differential electron-capture cross sections of the two molecular states, into which electrons are ``fed'' by the surrounding surface structure.