Dynamics of the reaction of O− with D2 at low collision energies: reagent rotational energy effects

The effects of reagent rotational excitation on the dynamics of the O−+D2 particle transfer reaction are investigated in a crossed molecular beam experiment. Vibrational-state-resolved angular distributions are measured at collision energies of 0.22, 0.25, and 0.37 eV as a function of the rotational temperature of the D2 reactant gas, which ranges between 58 and 425 K. When the rotational temperature of the D2 is lowered, the angular distributions become more strongly backward scattered with a tighter, more intense peak at 180°. In addition, the relative amounts of forward and sideways scattering are decreased. We interpret these product angular distributions as reflecting specific geometries required for passage through the critical transition state region of the potential energy surface where the particle transfer and electron detachment channels compete. Reagent rotational motion correlates to motion in the transition state that is selectively coupled to electron detachment. The more rotational energy present in the reagents, the smaller the probability that the complex remains linear and produces backward-scattered OD−. The product vibrational-state distributions change by only a few percentage points with the variation in rotational energy. While the small difference in the amount of rotational energy present in this system strongly influences the mechanism of particle transfer, it may not significantly alter the number of trajectories leading to that channel. The dynamics of this system are discussed in terms of a local complex potential describing nuclear motion in the critical region of the potential surface.