RESONANT SUBSTRATE-MEDIATED PHOTODISSOCIATION OF ADSORBED MOLECULES - O-2/GRAPHITE

We report the photon stimulated desorption of O- ions from thin films (1-10 monolayers) of O2 condensed on graphite in the photon energy range 15-35 eV. The negative ion yield exhibits two peaks, at 25 and 30 eV, quite unlike the gas phase photodissociation cross section. Our results indicate that the 25 eV feature is due to dipolar dissociation of condensed O2 molecules driven by photoelectrons generated in the substrate. We propose that the resonant nature of this feature arises from specific interband transitions in the graphite bandstructure, highlighting the importance of final state, as well as initial state, effects in substrate-mediated photodesorption of adsorbed molecules.