MECHANISTIC STUDIES OF THE PHOTODISSOCIATION OF PHYSISORBED O-2/GRAPHITE

We have investigated the vacuum ultra-violet photodissociation of molecular O2 physisorbed on graphite using synchrotron radiation with photon energies in the range 13-35 eV. The yield of desorbed O+ ions shows a threshold at approximately 19.5 eV and resonances at 24.5 eV and 28.5 eV, contrasting strongly with the gas phase photodissociation cross section. The principal mechanism of O+ production appears to be dipolar dissociation driven by photoelectrons generated in the substrate. An enhancement in the detection efficiency of low-energy ions, compared with previous measurements, has identified an additional dissociation mechanism leading to the desorption of O- ions. Specifically, the results support a channel for low-energy O- desorption, attributed to the dissociative attachment of photoelectrons, again generated in the graphite substrate, to the physisorbed O2 molecules.