Comparative study of the primary photochemical mechanisms of nitric oxide and carbonyl sulfide on Ag(111)

Detailed studies of the primary photochemical mechanism of two quite differ ent adsorbates, OCS and NO, on a common substrate, AE(III), have been made. Irradiation of OCS on Ag(lll) at wavelengths shorter than 500 nm leads to dissociation into COg and S-a. For a Ag(111) surface saturated with NO, irr adiation at all wavelengths studied results in desorption of both NO and N2 O, The relative photochemical cross sections for the reactions have been me asured as a function of the wavelength, polarization, and angle of incidenc e of the radiation. The polarization dependence was measured at four wavele ngths and three angles of incidence, and the results were compared with pre dictions for both adsorbate and substrate localized excitation mechanisms. The results are similar for both adsorbates and consistent with a substrate excited mechanism. No evidence was found for a change in photochemical mec hanism with wavelength for either adsorbate. The wavelength dependence of t he photochemical cross sections was studied in greater detail, between 280 and 600 nm. For irradiation at wavelengths shorter than 400 nm, the wavelen gth dependence is essentially identical for each surface, strongly suggesti ve of a substrate excited mechanism. For the photodissociation, of OCS a th reshold at ca. 500 nm was observed, while for NO desorption. no threshold w as detected out to 600 nm. The wavelength-dependent cross sections were mod eled assuming the primary photochemical mechanism to be attachment of hot, sub-vacuum electrons generated by substrate absorption. For OCS, the cross section was modeled assuming an adsorbate attachment level at 3.2 eV and a nascent distribution of hot electrons. For NO, at least two adsorbate attac hment levels are required to reproduce the data and the effect of secondary hot electrons must be taken into account. It was concluded that all measur ements could be interpreted on the basis of the substrate-mediated hot elec tron attachment mechanism, but that the limited knowledge of adsorbate elec tronic structure hampered more definitive conclusions.