ROLE OF SUBSTRATE EXCITATION IN THE PHOTOCHEMISTRY OF DIOXYGEN ON AG(110)

The rates Of O2 photodissociation and photodesorption on Ag (110) under UV irradiaton have been measured with temperature programmed desorption and high-resolution electron energy loss spectroscopy. The alignment Of O2 adsorbed on Ag(110) permits polarization measurements at normal incidence. The rates are found to be independent of the azimuthal orientation of the electric field of the light. Dependences on angle of incidence are consistent with the angle dependence of metal absorption predicted by Fresnel's equations. These observations rule out direct intra-adsorbate excitation and suggest a substrate mediated mechanism. Similar photoexcitation thresholds suggest that both channels are activated by a common process. A model is proposed whereby substrate charge transfer induces dissociation. Dissociating adatoms, in competition with molecular oxygen for adsorption sites, induce molecular desorption. This model reproduces the dependences on initial O2 coverage and extent of irradiation for both channels and allows calculation of the photodissociation cross section.