Dynamics of the OH−+D2 isotope exchange reaction: Reactive and nonreactive decay of the collision complex

Abstract

Reactive and nonreactive collisions between OH− and D2 are investigated over the collision energy range from 0.27 to 0.67 eV by the method of crossed molecular beams. The angular and energy distributions measured for the isotope exchange reaction are quite similar at all collision energies, indicating that the collision dynamics are relatively insensitive to energy over this range. Although the exchange reaction involves the formation of an intermediate complex ion in which bond rearrangement takes place, the OD− products are primarily backward scattered with only low intensity scattering appearing in the forward direction. The forward scattered products do contribute proportionately more intensity to the complete differential cross section as the collision energy increases, suggesting that both direct and collision complex mechanisms are responsible for reaction. The angular distributions are interpreted in terms of the osculating model for chemical reaction occurring in a fraction of the rotational period of the intermediate complex, augmented by a small forward scattered direct component. This model suggests that the complex lifetime is approximately 150 fs at 0.27 eV. When the OD− product is backward scattered, little internal energy is found in either product. A simple impulsive model explains the lack of internal excitation based on the geometry of the transition state of the complex. More of the available energy is deposited into product internal modes when the intermediate complex lives longer and OD− is forward scattered. The collision energy can be redistributed into bending modes of the complex, which impart angular momentum to the fragments when the complex dissociates. The recoil energy distributions for the nonreactively scattered OH− show strong similarities to the reactively scattered OD− distributions and are useful in probing the origin of the product rotational excitation. Structure in the recoil energy distributions corresponding to inelastically scattered OH− indicates the preferred deposit of available energy into specific rotational modes. © 2000 American Institute of Physics.