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 rea ction are studied theoretically. Based on a new ab initio potential energy surface, the nuclear dynamics is followed by grid methods and wave packet p ropagation techniques. All five planar degrees of freedom are included in t he calculation, for all three different isotopomers. The experimental photo electron spectra by Lineberger and co-workers are very well reproduced; upo n a small adjustment of the calculated anionic equilibrium geometry the agr eement becomes excellent. The vinylidene survival probability for broadband photodetachment exhibits three different time regimes, the longest of whic h points towards an unusual stability of this reactive intermediate. The la tter finding is corroborated by the calculated state-specific lifetimes whi ch exceed previous estimates in the literature by similar to3 orders of mag nitude. These findings are found to be reconfirmed when taking the discrete level structure of vibrationally highly excited acetylene into account. Th ey amount to heavy barrier recrossing effects in this isomerization reactio n and lend strong support to the interpretation of CEI experiments on vinyl idene by Levin where this species has been identified similar to3 mus after its formation. (C) 2001 American Institute of Physics.