Dynamics of the reaction of O− with H2O : Reactive and nonreactive decay of collision complexes

Abstract

We present a study of reactive and nonreactive collisions between O− and H2O over the collision energy range from 0.67 to 1.07 eV. Kinetic energy analysis of the O− scattered nonreactively from H2O shows two components: The first arises from direct scattering and is nearly quasielastic, while the second occurs at significantly lower barycentric energies and corresponds to O− ejected without reaction from electrostatically bound O−⋅H2O complexes formed by approaching reagents. This latter flux is significantly more intense than the reactive OH− flux. The kinetic energy distributions for the low energy O− nonreactive flux are in qualitative agreement with statistical phase space theory, although recoil distributions that model the exit channel by an r −4 potential underestimate the kinetic energy release. The reactive flux distributions show a strong energy dependence. At the lowest collision energy, the OH− is produced through two pathways, the first involving the participation of a complex living a fraction of a rotational period, the second producing OH− strongly backward scattered and with a much broader kinetic energy distribution. With increasing collision energy, the complex contribution to the scattering falls off rapidly, and product formation moves from the backward hemisphere to the forward direction. The angular distribution asymmetries at the lowest collision energies can be interpreted in terms of the osculating model for chemical reactions taking place in a fraction of a rotational period of the intermediate complex. This model suggests that the complex lifetime is ∼250 fs at collision energies between 0.7 and 0.8 eV, a result in good agreement with Rice–Ramsperger–Kassel–Marcus (RRKM) calculations. The kinetic energy distributions at these energies are in good agreement with statistical phase space theory calculations. At the highest collision energies, still below the threshold for impulsive stripping collisions, the OH− product is scattered sharply forward with a broad kinetic energy distribution peaking near 0.3 eV. We interpret the high energy dynamics as direct, but still involving significant interaction among all four atoms. The rapid variation in dynamics over a narrow collision energy range is attributed to the heavy–light–heavy mass combination of this system.