A crossed molecular beam study of the reaction O(1D) + HI -> IO + H

The angular and velocity distributions of the IO product from the reaction O(1D) + HI have been obtained in crossed beam experiments at collision energies, Ec, of 4.7 and 13.6 kcal/mol. The product center-of-mass angular distribution is found to be nearly backward - forward symmetric, with forward scattering favored. The forward to backward scattering ratio increases with increasing Ec, from which it is deduced that the reaction occurs via an intermediate complex having a lifetime comparable to its rotational period. Within the "osculating" model for chemical reactions, the complex lifetime is estimated to be 4 and 1 ps at low and high Ec, respectively. A large fraction (0.55 at low Ec and 0.46 at high Ec) of the total available energy was found to be released into product translational energy. Comparison with the dynamics of the analogous reactions O(1D) + HCl and O(1D) + HBr is carried out: similarities as well as differences are noted and discussed in terms of the potential energy surfaces for reaction. From comparison of signal intensities and the previously estimated lower limits to the branching ratio of the relative cross sections for XO + H and OH + X channels in the O(1D) + HX (X = Cl, Br) reactions, it is concluded that the H-displacement pathway accounts for a significant fraction of the overall reaction also in the case of O(1D) + HI.