Dynamics of the reaction O(³P) + HBr: Experimental investigation and theoretical modeling

The reaction O(³P) + HBr → OH(X²Π) + Br has been investigated experimentally. Two distinct approaches were pursued, differing primarily in the method of O(³P) atom production. The first involved crossing a pulsed, supersonic free jet of HBr with an effusive jet of O(³P) atoms produced by a microwave discharge in O₂, and the second employed laser photolysis of NO₂ in a bulk mixture with HBr. The two methods gave rather similar OH product state distributions with a strong vibrational inversion (v″ = 0, 1, 2 in the ratio 0:9:1) and substantial rotational excitation extending to the limit of available energy. The dynamics appear consistent with expectations for the kinematically constrained reaction heavy + light-heavy → heavy-light + heavy. Evidence was found for a contribution from reaction of (HBr)_n van der Waals clusters in the crossed-beam experiments, and more authentic detailed distributions are believed to be obtained via the laser photolysis approach. Nonstatistical populations of the OH fine structure states were observed. A minor channel (∼6%) producing spin-orbit excited Br(²P_1/2) is proposed as an explanation for an apparent anomaly in the OH(v″=1) rotational distribution. The experimental results for the O(³P) + HBr system are compared with quasi-classical trajectory calculations on a semiempirical London-Eyring-Polanyi-Sato potential energy surface, which Broida, Tamir, and Persky derived to optimize agreement between calculated and observed kinetic data. Good agreement is found between the predictions of these calculations and the experimental observations, particularly in the fractional partitioning of the energy available to the products into translation, vibration, and rotation. The O(³P) + HBr system is contrasted with previously studied reactions of O(³P) with organic molecules, in which the OH product exhibits little rotational excitation. The disparate behavior of the two systems is rationalized by consideration of the different angular dependence of model potential surfaces which satisfactorily reproduce the observed dynamics in each case.