Collisional probing of the transition-state structure of a bimolecular reaction

The total cross section sigma(B)(E) for the Ca(D-1) + HBr --> CaBr(B-2 Sigma(+)) + H reaction as a function of collision energy has been measured using crossed atomic and molecular beams. The maxima exhibited by sigma(B)(E) with increasing energy are attributed to the opening of successive bending vibrational reaction channels that proceed via a [Ca(D-1)... Br ... H]double dagger transition state. A dynamical model for the reaction may be constructed in terms of Landau-Zener probabilities for curve crossing at two locations on the reaction path, coupled with a preference for consumption of transition-state vibrational energy.