Laser-induced fluorescence spectra and the observation of quantum beats in the à 2Πi−X̃ 2Πi transition of the HCCS radical

LIF spectra of the à 2Πi−X̃ 2Πi transition of the HCCS radical were observed in a supersonic jet generated by using a pulsed-discharge nozzle. Adiabatic cooling yielded definitive rotational assignments for ≈ 40 vibronic bands. The fluorescence from single rovibronic levels exhibited a quantum beat in most of the observed bands. The number of beat components increased gradually with internal energy, approaching the limiting case for a ‘large molecule’. The observed quantum interference is ascribed, based on the estimated densities of background states, to the internal conversion coupling with highly excited vibrational levels in the X̃ 2Πi state.