State-resolved dissociation dynamics of triplet acetaldehyde near the dissociation threshold to form CH3+HCO

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 CH3 + 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 # 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 #, similar to 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. (C) 2000 American Institute of Physics. [S0021-9606(00)00704-2].