Mode specificity and energy dependence of the dynamics of single rovibrational levels of S1 acetaldehyde in a supersonic jet

The dynamics of selected single rotational S1 levels of low lying vibrational levels near the π*←n (A″←A′) electronic origin of CH3CHO (T′00=29 771 cm−1) and CH3CDO (T′00=29 820 cm−1) have been examined. The aspects that have been studied include the effects of three low frequency vibrational modes; ν′10 (CCO deformation), ν′14 (CH out-of-plane bend), and ν′15 (CH3 torsion), the effect of rotation, the effect of deuterium substitution, and the similarity as well as any differences with the S1 dynamics of formaldehyde. The fluorescence decay times measured in the supersonic jet show (i) single exponential decay, (ii) lifetimes in the range 225–95 ns for CH3CHO and 1.52–1.02 μs for CH3CDO, (iii) ν′10 and ν′14 are effective in promoting the S1 decay, but ν′15 is not, (iv) an underlying trend of decreasing lifetimes with increasing E′vib, (v) no evidence of explicit dependence on the rotational state at low energies, and (vi) no evidence for quantum beats which would possibly be associated with strong S1–T1 coupling. The observation of a single exponential decay from a cold single rotational level is decidedly different from the recent report of dual exponential decay behavior at room temperature.