PREDISSOCIATION MECHANISM AND SPIN-ROTATION CONSTANT OF THE HCO $\tilde{B}^{2} A'$ STATE

Abstract

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