PHOTOLYSIS OF TETRAMETHYLSILANE NEAR THE ABSORPTION ONSET - MECHANISM AND 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 and a methane elimination process with the concomitant formation of dimethylsilaethylene (Phi=0.17+/-0.04). Other very minor primary processes occur, with quantum yields of the order of Phi 5x10(-3), but their nature 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 the Si=C double bond occurs preferentially at the Si site. Satisfactory material balance was obtained indicating that the products were mostly recovered. A number of relative rate constants were determined. Reactions 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 channels occur from different electronic states. The lack of dependence of the CH4 quantum yield on the experimental parameters (liquid or gaseous phase, etc.) suggests a decomposition from a strongly predissociating state, which is identified with the lowest excited singlet state, while the Si-C bond breaking process is thought to occur from the triplet state. Molecules which reach the ground state live sufficiently long so that deactivation competes successfully with decomposition.