We report the photon stimulated desorption of O-ions from thin films ( 1-10 monolayers ) of 02 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 photoeleclrons 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 substratemediated photodesorption of adsorbed molecules.
Recent studies have shown that the adsorption of molecules on surfaces opens new channels for light absorption and therefore introduces entirely new photochemical pathways compared with the gas phase [ 1-3 ]. The adsorbate may itself absorb a photon resulting in direct photodissociation or photodesorption , or dissociation may occur by an indirect, substrate-mediated mechanism. Proposed indirect mechanisms include electronic energy or charge transfer from substrate to adsorbate . Charge transfer mechanisms involving hot electrons (i.e. electron energy less than the work function) have been invoked in the interpretation of a number of desorption and dissociation experiments. The recent observation of negative ion desorption by gives perhaps the most direct evidence for charge transfer photodissociation induced by hot electrons. In general, determination of the energy dependence of the relevant cross sections is a