Barrier recrossing in the vinylidene-acetylene isomerization reaction:: A five-dimensional ab initio quantum dynamical investigation

The spectroscopy and dynamics of the vinylidene-acetylene isomerization reaction are studied theoretically. Based on a new ab initio potential energy surface, the nuclear dynamics is followed by grid methods and wave packet propagation techniques. All five planar degrees of freedom are included in the calculation, for all three different isotopomers. The experimental photoelectron spectra by Lineberger and co-workers are very well reproduced; upon a small adjustment of the calculated anionic equilibrium geometry the agreement becomes excellent. The vinylidene survival probability for broadband photodetachment exhibits three different time regimes, the longest of which points towards an unusual stability of this reactive intermediate. The latter finding is corroborated by the calculated state-specific lifetimes which exceed previous estimates in the literature by similar to3 orders of magnitude. These findings are found to be reconfirmed when taking the discrete level structure of vibrationally highly excited acetylene into account. They amount to heavy barrier recrossing effects in this isomerization reaction and lend strong support to the interpretation of CEI experiments on vinylidene by Levin where this species has been identified similar to3 mus after its formation. (C) 2001 American Institute of Physics.