Laser-induced fluorescence from the predissociating formyl radical. 1. Mechanism for the predissociation of the A2A″ state

Fluorescence excitation spectra of the HCO Ã(0,11⁰,0)-X̃(0,0¹,0) and Ã(0,9⁰,0)-X̃(0,0¹,0) transitions and the DCO Ã-(0,15°,0)-X̃(0,0¹,0) transition are reported. Spectral simulations of each vibronic band show that all rotational transitions are lifetime broadened, with rotational line widths of ∼0.5 and ∼0.8 cm^-1 for HCO and DCO, respectively. These values are constant up to N′ ≈ 10 and then increase with an N′²(N′ + 1)² dependence. The lifetime-broadening mechanism, which is nonradiative in nature, is due to predissociation to form H(D) + CO. The increase in line width is accompanied therefore by a reduction in fluorescence intensity. The origin of the homogeneous electronic predissociation mechanism at low N′ is unknown since no explanation for such a process currently exists. The N′²(N′ + 1)² dependence strongly implies K-type resonance as the dominant process for the heterogeneous electronic predissociation at high N′. The more facile rate of predissociation of DCO over HCO precludes quantum mechanical tunnelling as an explanation of the observed line-width and intensity dependence on N′.