LASER PHOTODISSOCIATION DYNAMICS OF THIONYL CHLORIDE - CONCERTED AND STEPWISE CLEAVAGE OF S-CL BONDS

The photodissociation of thionyl chloride (Cl2SO) is of interest as a model system to study three-body fragmentation processes, which can oc cur either in concert or stepwise. The photodissociation of this tetra tomic molecule at 193 and 248 nm has been studied by laser induced flu orescence spectroscopy of the nascent SO fragment on the B3SIGMA--X3SI GMA- transition in the region of 237-295 nm. Photolysis of Cl2SO at 19 3 nm leads to an inverted vibrational distribution for the nascent SO( X3SIGMA-) with a population maximum at v'' = 2. The quantum yield, PHI (SO(X))193 nm = 0.73 +/- 0.10, has been measured by comparison of the SO(X3SIGMA) produced from SO2. The results indicate a concerted three- body fragmentation mechanism as the primary dissociation channel. A Fr anck-Condon/golden rule model elucidates the geometry prior to the fra gmentation and suggests a direct dissociation mechanism. The rotationa l- and spin-state distributions have been measured from the rovibronic ally resolved spectra to support our model of the detailed dissociatio n mechanism. At 248 nm, the nascent vibrational distribution was found to be bimodal. The vibrational state population distribution in v'' = 0-2, which accounts for most (approximately 94%) of the nascent SO(X3 SIGMA-) population, was found to be thermal (T(vib) = 1000 +/- 200 K), suggesting a stepwise fragmentation process. About 6% of the nascent SO population has been observed in other vibrational levels (v'' = 3-7 ) and most likely originates from the molecular elimination of Cl2 fro m Cl2SO.