The dynamics of the reaction of O(1D) with HBr studied by crossed molecular beams and time-resolved Fourier transform spectroscopy

Abstract

The reaction O(1D) + HBr has been investigated by the crossed molecular beams and infrared chemiluminescence methods in an effort to characterize the dynamics of both possible reactive channels. The angular and velocity distribution of the BrO product from the O(1D) + HBr → BrO + H pathway have been obtained in crossed beam experiments at collision energies, Ec, of 5.0 and 14.0 kcal/mol. The product center-of-mass angular distribution is found to be almost backward–forward symmetric at both Ec, with backward scattering being slightly favored, from which it is deduced that two processes contribute to this channel: a dominant one occurring via formation of a long-lived complex, following O(1D) insertion, and another one occurring via direct abstraction of the halogen atom and giving rise to a rebound dynamics. The large fraction (≈50%) of available energy released into translation indicates the existence of a potential barrier for H-displacement in the exit channel. From energy and angular momentum conservation arguments, it is inferred that BrO is formed rotationally very hot in the lowest vibrational levels of both 2Π3/2 and 2Π1/2 electronic states. The initial vibrational distribution of the OH product from the O(1D) + HBr → OH + Br channel has been measured using fast time-resolved Fourier transform spectroscopy. The vibrational distribution is strongly inverted, from which it is deduced that the HOBr intermediate dissociates very rapidly, before energy randomization occurs. A lower limit to the branching ratio of the relative cross sections for the BrO + H and OH + Br channels is derived (σ(BrO + H)/σ(OH + Br) ≥ 0.16 ± 0.07) and compared to recent bulk work. The dynamical results for the overall reaction are discussed with reference to the relevant singlet and triplet potential energy surfaces and possible molecular configurations involved. Comparison with the dynamics of the ground state reaction O(3P) + HBr → OH + Br is carried out also, to examine the effect of electronic excitation on the dynamics of the reactions of atomic oxygen with hydrogen halides.