Nuclear spin dependence of the reaction of H₃⁺with H₂. I. Kinetics and modeling

The chemical reaction H₃⁺ H₂ → H ₂ H₃⁺ is the simplest bimolecular reaction involving a polyatomic, yet is complex enough that exact quantum mechanical calculations to adequately model its dynamics are still unfeasible. In particular, the branching fractions for the identity, proton hop, and hydrogen exchange reaction pathways are unknown, and to date, experimental measurements of this process have been limited. In this work, the nuclear-spin-dependent steady-state kinetics of the H₃⁺ H₂ reaction is examined in detail, and employed to generate models of the ortho:para ratio of H₃⁺ formed in plasmas of varying ortho:para H₂ ratios. One model is based entirely on nuclear spin statistics, and is appropriate for temperatures high enough to populate a large number of H ₃⁺ rotational states. Efforts are made to include the influence of three-body collisions in this model by deriving nuclear spin product branching fractions for the H₅₊ H₂ reaction. Another model, based on rate coefficients calculated using a microcanonical statistical approach, is appropriate for lower-temperature plasmas in which energetic considerations begin to compete with the nuclear spin branching fractions. These models serve as a theoretical framework for interpreting the results of laboratory studies on the reaction of H₃⁺ with H₂.