Inelastic collisions of OH(X 2Π) with para-H2: Λ-doublet and hyperfine-structure transitions

In this article we report Λ-doublet and hyperfine-structure resolved close-coupled and coupled-states cross sections for collisions of OH (X ²Π) with para-H₂ at thermal collision energies relevant to kinetic modelling of OH emission in astrophysical sources. Our calculations predict a degree of selective excitation of the Λ-doublet levels in agreement with earlier work of Schinke and Andresen a a higher collision energy. That is, collisions of OH with H₂ lead to preferential population of the lower Λ-doublet (ɛ=+1,e) in the Ω=3/2 manifold, while the upper Λ doublet (ɛ=-1,f) is preferentially populated in the Ω=1/2 manifold. In contrast to a purely stastistical model, we observe here that, for a given JΩɛ→JΩɛ' transition, the hyperfine resolved cross sections are consistent with a strong ΔF=0 collisional propensity rule. This collisional propensity is a manifestation of a fundamental tendency to conserve the orientation of the rotational angular momentum vector during collisions of molecules with closed-shell targets. This propensity rule implies that cross sections for transitions across a hyperfine doublet will be small.