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. O2 adsorbs molecularly with the O-O axis parallel to the surface and aligned along the Ag[110BAR] azimuth. This alignment permits polarization measurements to be made at normal incidence. Photolysis 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. Taken together, these observations rule out direct photoexcitation and suggest substrate excitation followed by dissociative electron capture as the photodissociation mechanism. Similar energy thresholds (2.8 eV) suggest that both channels are activated by a common process. A reaction model is proposed whereby substrate charge transfer induces dissociation. Dissociating adatoms, in competition with dioxygen for available binding sites, force some desorption of the latter. Analysis using 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-dependent photolysis rates and wavelength dependences for O2 on Pt(111) suggest that direct intraadsorbate excitation contributes significantly to photodesorption and may participate in photodissociation.