State-resolved dissociation dynamics of triplet acetaldehyde near the dissociation threshold to form CH31HCO

We studied the state-resolved dynamics of S1 acetaldehyde to product channels with quantum-beat spectroscopy. Two bands near the threshold of dissociation to radical products CH31HCO in a supersonic jet, displaying most quantum-beat features, are recorded with resolution 0.025 cm. Evaluated on the basis of a simple asymmetric rotor, the origins of these two bands 140 150 1 and another denoted # are 31 275.045~1! and 31 523.263~1! cm; effective rotational constants of excited state are A55.7883(1), 5.0408~3!, B50.33269(2), 0.32320~2! and C50.31026(2), 0.32091~2! cm, 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 #, ;125 cm below the dissociation threshold, increases with increasing total angular momentum J whereas level 1415 that is 375 cm 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. @S0021-9606~00!00704-2#