Lifetimes of vibrational levels in the B̃2A′ state of HCO

We measure the fluorescence decays for low-lying rotational states in the first eight vibrational levels of HCO B̃2A′. Some long-lived decays exhibit weak quantum beats that we attribute to a higher-order hyperfine perturbation in the B̃ state. We estimate fluorescence quantum yields by combining measured fluorescence intensities and a calculated electronic transition moment. The estimated nonradiative decay rates increase with increasing vibrational excitation and exhibit mode specificity. We discuss possible vibronic coupling mechanisms, including spin-orbit coupling to quartet states and predissociation via the Ã2Π(2A″) and X̃2A′ states.