To obtain insights into the photophysical properties of collision-free T-1((a) over tilde (3)A(2)) thiophosgene, Cl2CS, the cavity ringdown (CRD) spectrum of the T-1<-- S-0 absorption system was recorded under supersonic jet conditions and compared with the corresponding excitation spectrum of the total emission. It was found that none of the T-1<-- S-0 bands in the CRD spectrum appears in the excitation spectrum, indicating that the T-1 thiophosgene decays almost exclusively by the nonradiative T-1--> S-0 intersystem crossing (ISC). An estimation of the T-1 nonradiative lifetime was made using the T-1-S-0 spin-orbit coupling and the Franck-Condon factors for the T-1--> S-0 ISC based on the ab initio equilibrium structures and vibrational frequencies for the T-1 and S-0 states, computed at the MP2/6-31G(d,p) and MP4/6-31G(d,p) level of theory. The nonradiative life, calculated as the 1/e of the survival probability, is approximately 20 ps for barrier heights of 770-845 cm(-1) and out-of-plane angles of 32.07 degrees-32.69 degrees. (The thiophosgene adopts a pyramidal conformation with the C=S bond bent from the ClCCl plane by about 32 degrees.) The computed lifetime is comparable to the lower-limit lifetime of similar to 50 ps obtained from the quantum yields Phi(P)less than or equal to 10(-3)Phi(F) with Phi(F)approximate to 1.0 and the measured S-1 fluorescence lifetime of 4 mu s. The short T-1 nonradiative lifetime of thiophosgene can be attributed to the large matrix elements of the T-1-S-0 spin-orbit coupling (V=150 cm(-1)) and the strong out-of-plane deformation of the T-1 state relative to the planar S-0 state, that provides the large Franck-Condon factors for the T-1--> S-0 ISC. It is proposed that a similar pyramidal deformation is also responsible for the absence of phosphorescence from T-1 thiocyclobutanone. Consistent with this supposition, T-1 cyclopentanone, which is nearly planar at the thiocarbonyl carbon center, exhibits strong phosphorescence under similar experimental conditions.