SELECTIVE RESONANCE POPULATION IN ELECTRON-SCATTERING BY ADSORBED MOLECULES

We have investigated vibrational excitation of physisorbed O2 by electron impact in the 414-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 u4 state (at 8.5 eV) and the u2 state (at 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. © 1990 The American Physical Society.