Core-induced photofragmentation of acetonitrile adsorbed on Au(111) and Pt(111)

Photodissociation of acetonitrile (CH3CN) condensed, physisorbed on Au(111) , and chemisorbed on Pt(111), has been studied at the C(1s) and N(1s) edges in H+ yield and/or in total positive ion yield. The H+ yield C(1s) absorpt ion experiment on the condensed acetonitrile shows a selective fragmentatio n of the C-H bond at the C 1s -->pi*CH3 excitation, that is still observed when acetonitrile is physisorbed on Au(111) and chemisorbed on Pt(111). Tra nsitions to the pi*C=N state give rise to unselective fragmentations, which are strongly quenched by charge transfer from the substrate when acetonitr ile is chemisorbed on Pt(111). The pi(perpendicular to)*C=N level that plun ges into the Pt(111) is the most affected and vanishes in the total ion yie ld measurement. The quenching is weaker on the pi(parallel to)*C=N which is parallel to the surface, and thus is less hybridized with the metal than t he pi(perpendicular to)*C=N. For acetonitrile physisorbed on Au(111), the i nfluence of the surface is limited to the quenching of the desorption of th e heaviest outgoing fragments. (C) 2000 American Institute of Physics. [S00 21-9606(00)70502-2].