PHOTOLYSIS OF TETRAMETHYLSILANE NEAR THE ABSORPTION ONSET - MECHANISMAND PHOTOPHYSICS

The excitation of tetramethylsilane (Me(4)Si) into its lowest excited Rydberg state is followed by two main decomposition channels: a simple Si-C bond breaking process with a quantum yield of Phi=0.45+/-0.05 an d a methane elimination process with the concomitant formation of dime thylsilaethylene (Phi=0.17+/-0.04). Other very minor primary processes occur, with quantum yields of the order of Phi 5x10(-3), but their na ture could not be identified with certainty. The reactions leading to the stable products are dominated by radical-radical processes and by radical addition reactions to Me(2)SiCH(2). The addition reaction to t he Si=C double bond occurs preferentially at the Si site. Satisfactory material balance was obtained indicating that the products were mostl y recovered. A number of relative rate constants were determined. Reac tions in the presence of NO, MeOH, GeH4 and SF6 were also studied. An explanation of the photophysics by a three-state model was attempted. From the experiments, it was concluded that the two decomposition chan nels occur from different electronic states. The lack of dependence of the CH4 quantum yield on the experimental parameters (liquid or gaseo us phase, etc.) suggests a decomposition from a strongly predissociati ng state, which is identified with the lowest excited singlet state, w hile the Si-C bond breaking process is thought to occur from the tripl et state. Molecules which reach the ground state live sufficiently lon g so that deactivation competes successfully with decomposition.