THE VACUUM-ULTRAVIOLET PHOTODISSOCIATION OF SILANE AT 125.1 NM

We observe the photodissociation of SiH4 in a pulsed molecular beam after excitation with 125.1 nm vacuum ultraviolet light generated via resonant four-wave mixing in mercury vapor. Ultraviolet radiation from a Nd:YAG/dye laser combination ionizes the neutral photodissociation fragments and a time-of-flight mass spectrometer detects the ions. The photodissociation signal consists entirely of silicon atoms and silylidyne (SiH) in its first electronically excited state. We see no silylene (SiH2) Or silyl radicals (SiH3). Thus, the photodissociation cleaves almost all of the silicon-hydrogen bonds, but the energetics of the dissociation require the production of at least one hydrogen molecule per dissociation, event. These results imply that the high energy content of the initially excited Rydberg state prevents formation of the silylene and silyl radicals in stable vibronic states and that dissociation pathways exist that connect the Rydberg state directly to the, corresponding silicon atom and silylidyne asymptotes. These pathways are likely to exist because of Jahn-Teller distortion from the initial T(d) symmetry. Very little of the available energy appears as kinetic energy of the fragments but rather as electronic excitation of the products. Our results differ from those of earlier studies that concluded silylene and silyl are the principle products.