PHOTOINDUCED PATHWAYS TO DISSOCIATION AND DESORPTION OF DIOXYGEN ON AG(110) AND PT(111)

The rates for photodissociation and photodesorption of dioxygen on Ag(110) under UV irradiation have been measured with TPD and HREELS. The bonding of O2 to 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, force the 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. In contrast, angle and wavelength dependences for O2 on Pt(111) suggest the participation of direct excitation in the photochemistry.