Photodesorption of NO from chemically modified Ni(111) surfaces

NO photodesorption from the oxygen-covered, oxidized, and sulfur-saturated Ni(111) surfaces was studied. Temperature programmed desorption effectively discriminates between NO species adsorbed on Ni metal sites and on Ni sites modified by O or S, making the study of photodesorption from each site possible. Oxygen- and sulfur-modified Ni(111) surfaces both exhibit enhanced photodesorption cross sections compared to a clean Ni surface. Both the direct observation of NO(g) photodesorption and postirradiation thermal desorption measurements of NO coverage have been carried out. The photodesorption process is first order in photon flux and in NO coverage within a given chemisorption state. The photon-energy dependence of the NO photodesorption cross section from oxidized and S-saturated Ni(111) surfaces was measured in detail over a photon energy range of 1.5–3.8 eV. For both oxidized and sulfur-saturated Ni(111), the threshold for NO photodesorption is ∼1.5 eV. In addition, the photodesorption cross sections are dependent on the adsorption states of NO on the modified Ni(111) surfaces. The mechanisms of NO photodesorption are discussed and it is proposed that NO photodesorption is probably induced by the valence electron excitation from the occupied NO 2π*-d state to the unoccupied NO 2π*-d state. However, a hot electron model originating from the substrate cannot be excluded. The quenching of electronically excited NO on the clean Ni(111) surface is efficient.