PREDISSOCIATION MECHANISM AND SPIN-ROTATION CONSTANT OF THE HCO (B)OVER-TILDE(2)A' STATE

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 B 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 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 B̃ - X̃ transitions. The intensity distribution of the two spin states observed implies a negative value of the spin-rotation parameter for the B state, in contrast with the ground electronic state. © 1995 American Institute of Physics.