Photofragments CH3(X 2A2″)+HCO(X 2A′) from acetaldehyde: Distributions of rotational states and preferential population of K doublets of HCO

The relative rotational populations of HCO dissociated from acetaldehyde excited to the S1 state at photolysis energies 31 412, 31 771, 32 165, 32 728, and 33 239 cm−1 are obtained. Fluorescent spectra B̃ 2A′−X̃ 2A′(0,0) of HCO were detected; the intensity is corrected for both the fluorescent quantum yield and the effect of axis switching to obtain the populations of rotational states of HCO. All measured populations of rotational states of HCO, displaying a Gaussian-type pattern, cannot be described according to a Boltzmann distribution. The average energy partitioned to rotation of HCO along the b and c axes is [(270–465)±25] cm−1, about 11% of the available energy, but the average energy partitioned along the a axis remains the same (32±2) cm−1 for all photolysis energies. The rotational energy of HCO is less than that predicted according to an impulsive model calculated with the ab initio transition state structure. Preferential population of Ka=1 doublet states Nu1 and Nl1 is observed. The ratio of population varies with photolysis energy, with a maximum 12u1/12l1=3.7 near 31 800 cm−1. Distributions among the rotational state of HCO indicate that the energy released appears to be affected by the dynamics of the exit channel.