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 mea sured using crossed atomic and molecular beams. The maxima exhibited b y sigma(B)(E) with increasing energy are attributed to the opening of successive bending vibrational reaction channels that proceed via a [C a(D-1)... Br ... H]double dagger transition state. A dynamical model f or the reaction may be constructed in terms of Landau-Zener probabilit ies for curve crossing at two locations on the reaction path, coupled with a preference for consumption of transition-state vibrational ener gy.