PHOTOEXCITED PROCESSES AT METAL AND ALLOY SURFACES - ELECTRONIC-STRUCTURE AND ADSORPTION SITE

Photoexcited processes of NO and CO at photon energies ranging from 2. 3 to 6.4 eV are investigated on Pt(111), Ni(111) and Pt(111)-Ge surfac e alloys by reflection-absorption infrared spectroscopy and resonance- enhanced multiphoton ionization. The branching between three competiti ve processes of desorption, recapture and dissociation upon laser irra diation is dramatically changed on the three surfaces. On Pt(111), NO is either photodesorbed or photodissociated depending on the coverage, while NO is exclusively photodissociated on Ni(111). UV-photon irradi ation of NO on Pt(111)-Ge, on the other hand, induces only desorption of NO. Desorption of CO bound at the on-top site of Pt(111) is induced by laser irradiation. The electronic mechanism for photodesorption an d competitive branching is discussed in terms of the electronic struct ure of the substrate and the adsorbate. (C) 1997 Elsevier Science B.V.