PHOTODESORPTION OF CO, NO, AND O2 FROM MODIFIED NI(111) SURFACES

Photodesorption of CO, NO, and O2 from modified Ni(111) surfaces has been studied. Both direct observation of photodesorption and postirradiation thermal desorption measurements have been carried out. In each case, the photodesorption process is first order in photon flux and first order in the coverage within particular chemisorption states. The photon energy dependence of the photodesorption cross section was measured in the energy range from approximately 1.4 to 5.4 eV using a filtered Hg lamp. The CO photodesorption was observed only on the oxidized Ni(111) surface: no CO photodesorption was observed on the clean Ni(111) surface nor Ni(111) containing chemisorbed O and S. The photon energy dependence of the cross section for CO and O2 photodesorption resembles the optical absorption spectrum of NiO. We propose that, in the case of chemisorbed CO and O2 on NiO, photodesorption is induced by the NiO band gap interband transition. However, NO photodesorption occurs not only on the oxidized Ni(111) surface but also on the Ni(111) surfaces containing chemisorbed O and S. This observation suggests that NO photodesorption is induced by valence electron excitation in the NO-metal complex, but does not exclude the possibility of a contribution also from hot electron excitation in the substrate.