THE DYNAMICS OF THE REACTION OF O(D-1) WITH HBR STUDIED BY CROSSED MOLECULAR-BEAMS AND TIME-RESOLVED FOURIER-TRANSFORM SPECTROSCOPY

The reaction O(D-1) + HBr has been investigated by the crossed molecul ar beams and infrared chemiluminescence methods in an effort to charac terize the dynamics of both possible reactive channels. The angular an d velocity distribution of the BrO product from the O(D-1) + HBr --> B rO + H pathway have been obtained in crossed beam experiments at colli sion energies, E(c), of 5.0 and 14.0 kcal/mol. The product center-of-m ass angular distribution is found to be almost backward-forward symmet ric at both E(c), with backward scattering being slightly favored, fro m which it is deduced that two processes contribute to this channel: a dominant one occurring via formation of a long-lived complex, followi ng O(D-1) insertion, and another one occurring via direct abstraction of the halogen atom and giving rise to a rebound dynamics. The large f raction (approximate to 50%) of available energy released into transla tion indicates the existence of a potential barrier for H-displacement in the exit channel. From energy and angular momentum conservation ar guments, it is inferred that BrO is formed rotationally very hot in th e lowest vibrational levels of both (II3/2)-I-2 and (II1/2)-I-2 electr onic states. The initial vibrational distribution of the OH product fr om the O(D-1) + HBr --> OH + Br channel has been measured using fast t ime-resolved Fourier transform spectroscopy. The vibrational distribut ion is strongly inverted, from which it is deduced that the HOBr inter mediate dissociates very rapidly, before energy randomization occurs. A lower limit to the branching ratio of the relative cross sections fo r the BrO + H and OH + Br channels is derived (sigma(BrO + H)/sigma(OH + Br) greater than or equal to 0.16+/-0.07) and compared to recent bu lk work. The dynamical results for the overall reaction are discussed with reference to the relevant singlet and triplet potential energy su rfaces and possible molecular configurations involved. Comparison with the dynamics of the ground state reaction O(P-3) + HBr --> OH + Br is carried out also, to examine the effect of electronic excitation on t he dynamics of the reactions of atomic oxygen with hydrogen halides.