The effect of nonadiabaticity on the C⁺ + HF reaction

The chemistry of fluorine in the interstellar medium is particularly simple, with only a few key species and important reactions. Of the latter, the rate of the reaction of C⁺ ions with HF is not well established but is one of the key reactions that sets the relative abundance of HF and the CF⁺ ion, the two fluorine-bearing species that have been observed in interstellar clouds. The C⁺ + HF → CF⁺ + H reaction proceeds through a deeply bound HCF⁺ well. In this work, statistical methods, namely, the statistical adiabatic channel method originally developed by Quack and Troe and the quantum statistical method of Manolopoulos and co-workers, are applied to compute the total cross section as a function of energy for this reaction. This reaction proceeds on the ground 1² A' potential energy surface (PES), and there are also two non-reactive PES's, 1² A″ and 2² A', correlating with the C⁺(² P _1/2,3/2) + HF reactants. Two sets of scattering calculations were carried out, namely, a single-surface calculation on the 1² A' PES and the one in which all three PES's and the spin-orbit splitting of C⁺ are included in the description of the entrance channel. In the latter, reactivity of the spin-orbit excited ² P _3/2 level can be computed, and not just assumed to be zero, as in the single-state adiabatic approximation.