Nonadiabatic molecular collisions: Charge exchange and chemical reaction in the Ar+ + H2 system

The three adiabatic surfaces needed to describe Ar⁺ + H₂ in the nonrelativistic limit are calculated using the diatomics‐in‐molecules with zero overlap approach. Approximate nonadiabatic interaction terms which couple these surfaces are computed. Using these terms, a first‐order semiclassical calculation of the electronic transition probabilities is carried out. The results imply that all the important nonadiabatic transitions are localized in the reactant channel around an avoided crossing between the surfaces formed from combination of $\mathrm{Ar}{(}^1\mathrm{S)+}{\mathrm{H}}_2^{+}{(}^2{\Sigma }_g)$ and of Ar⁺(²P)+H₂(¹Σ). Because the results show that the transitions are induced by vibrational motion of the diatomic, any realistic study of the dynamics must include the full three dimensionality of the problem. We investigated the dynamics using the 3D trajectory surface hopping model proposed by Tully and Preston. Ar⁺ + H₂ (v=0) was studied at a relative energy of 3.36 eV. The predictions of the model for cross sections, average final translational energy, and average scattering angle agree well with experiment for both charge exchange and molecular rearrangement products. A discussion of the conceptual picture which emerges from this model is given.