Radiative and Nonradiative Processes in the Excited States of Jet-Cooled Oxalyl Chloride

Phosphorescence and fluorescence excitation spectra of oxalyl chloride (COCl)2 have been observed in a supersonic jet. Strong phosphorescence was observed for excitation of the band of the T1 3Au(nπ*) ← S0 transition. From a vibrational analysis, it is shown that the molecule has C 2h symmetry in the T1 excited state as well as in the S0 ground state. Small splittings due to 35Cl and 37Cl isotopes were found in several vibronic bands. For the excitation of the S1 band, phosphorescence with a short lifetime was observed, indicating fast intersystem crossing in the origin level. In the higher energy region, we could not observe phosphorescence but only weak fluorescence. It suggests existence of active nonradiative processes such as internal conversion and predissociation in the vibrationally excited levels of the S1 state. A single-vibronic-level (SVL) dispersed phosphorescence spectrum has also been observed for the excitation of the T1 CO stretching band. The spectrum was dominated by bands at the same wavelengths as those for the excitation. This clearly shows that intramolecular vibrational redistribution (IVR) takes place within the long triplet lifetime. Although oxalyl chloride is assumed to be a “small molecule”, the radiationless transitions have been found to be efficient in both the T1 and S1 states.