Abstract
We studied the state-resolved dynamics of S 1 acetaldehyde to product channels with quantum-beat spectroscopy. Two bands near the threshold of dissociation to radical products CH 3 +HCO in a supersonic jet, displaying most quantum-beat features, are recorded with resolution 0.025 cm -1 . Evaluated on the basis of a simple asymmetric rotor, the origins of these two bands 14 0 2- 15 0 1 and another denoted null are 31 275.045(1) and 31 523.263(1) cm -1 ; effective rotational constants of excited state are A = 5.7883(1), 5.0408(3), B = 0.33269(2), 0.32320(2) and C = 0.31026(2), 0.32091(2) cm -1 , respectively; large A value results from lack of consideration of torsional motion. For these two vibrational levels most rotational states (about 70 percent) display quantum-beat features attributed to coherently excited singlet-triplet eigenstates. The linewidth in transformed spectra for level null, ∼125 cm -1 below the dissociation threshold, increases with increasing total angular momentum J whereas level 14 2- 15 1 that is 375 cm -1 below shows a small linewidth independent of J. This is because correlation of the triplet state with dissociation to form radical products results in a decreased lifetime of the triplet state in the tunneling region. A systematic dependence on rotational quantum number implies Coriolis-induced vibrational coupling of triplet states to dissociating continuum on the exit side of the dissociation barrier. © 2000 American Institute of Physics.