A SPECTROSCOPIC VIEW OF INTERNAL-CONVERSION IN A SMALL POLYATOMIC MOLECULE - SUB-DOPPLER INTRACAVITY DYE-LASER SPECTROSCOPY OF THIOFORMALDEHYDE

Intracavity dye laser spectroscopy has been used to obtain sub-Doppler spectra of selected rotational lines in the A approximately 1A2-X approximately 1A1 4(0)1 band of thioformaldehyde with very high resolution and sensitivity. Many of the spectra show extra lines due to perturbations involving high vibrational levels of the ground state. Most of the perturbations observed for K(a)' = 0 and 4 are found to correlate well with previous observations of anomalously long single rotational level fluorescence lifetimes and reduced quantum yields [J. Dunlop and D. J. Clouthier, J. Chem. Phys. 93, 6371 (1990)]. S1-S0 interaction matrix elements of 0.001-0.006 cm-1 are found for levels involved in simple two level perturbations. The large number of small random perturbations by levels of the ground state is indicative of the first stages of the onset of quantum chaos in a small molecule. Some larger perturbations in the rotational structure are also observed; these are caused by additional local interactions with levels of the nearby triplet state.