Collisional Probing of the Transition‐State Structure of a Bimolecular Reaction

The total cross section σ B ( E ) for the Ca( 1 D) + HBr → CaBr(B 2 Σ + ) + H reaction as a function of collision energy has been measured using crossed atomic and molecular beams. The maxima exhibited by σ B ( E ) with increasing energy are attributed to the opening of successive bending vibrational reaction channels that proceed via a [Ca( 1 D) ⃛ Br ⃛ H] ‡ 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.