Predissociation mechanism and spin-rotation constant of the HCO B̃ 2A′ state

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Abstract
Formyl radicals produced from photolysis of acetaldehyde at 310 nm were supersonically cooled and detected via the B̃–X̃ transition using the laser‐induced fluorescence (LIF) technique. Spectra at 0.16 cm−1 resolution and fluorescence lifetimes of HCO B̃(0,0,0), (0,0,1), and (0,1,0) levels were measured. The observed lifetimes decrease rapidly with variation of the rotational quantum number Ka from 0 to 2 but slowly with the rotational quantum number N from 0 to 8. Experimental data indicate that the B̃ state is coupled to a predissociating state via an a‐type Coriolis interaction to account for the rotationally dependent lifetime for the low vibrational levels of the HCO B̃ state. Correction of the fluorescence quantum yield for individual rotational states is necessary to obtain accurate ground state populations by LIF when using the B̃–X̃ transitions. The intensity distribution of the two spin states observed implies a negative value of the spin‐rotation parameter for the B̃ state, in contrast with the ground electronic state.