Thiophosgene, a molecule tailor-made for testing fundamental theoretical concepts of radiationless transitions:: Intramolecular dynamics of S1Cl2CS

The collision-free S-1[(A) over cap (1)A(2)(n,pi*)] thiophosgene, with small (<3484 cm(-1)) and large (>9284 cm(-1)) excess vibrational energies, does not exhibit radiationless transitions to bound electronic states (S-0 and T-1), as evidenced by intense (A) over cap --> (X) over cap fluorescence with nearly constant lifetimes. The lack of radiationless transitions can be traced to the absence of the promoting mode for the (1)A(2) --> (1)A(1) internal conversion and the vanishingly small spin-orbit coupling and very small Franck-Condon factor for the (1)A(2)(npi*) --> (3)A(2)(npi*) intersystem crossing. In the intermediate range of excess energies (3484-9284 cm-(1)), fluorescence is not observed, consistent with the occurrence of predissociation. Quantum beat-modulated fluorescence decays, attributable to (1)A(2)(npi*)-(3)A(1)(pipi*) perturbations, have been observed for excitation of 1(1)2(1)3(1)4(1) and nearby S-1 vibronic levels. This places the b(3)A(1)(pipi*) state at about 20 350 cm(-1) above the ground electronic state, in excellent agreement with the existing estimate. Interestingly, the S-1 radiative lifetime is about an order of magnitude shorter in a supersonic jet as compared to the vapor phase at room temperature, indicating that the probability of the vibronically induced A(1)A(2)(npi*)- (X) over cap (1)A(1) radiative transition is significantly reduced at higher temperatures, due probably to Coriolis coupling.