Calculation of the Rate of the C+ + HF → CF+ + H Reaction: Implications for Fluorine Chemistry in the Interstellar Medium

Abstract

Abstract The chemistry of fluorine in the interstellar medium is primarily dependent on a few key reactions. Fluorine is unique among the elements because the F atom is the only light atom that can react exothermically with molecular hydrogen to form its hydride, HF. The only other fluorine-containing molecule of significant abundance is the CF+ ion, which is formed by the reaction of HF with C+ ions and is destroyed mainly by dissociative recombination. The observation of both HF and CF+ in molecular gas clouds offers an opportunity for a critical test of models for interstellar fluorine chemistry. Previous modeling calculations yielded an abundance of CF+ relative to that of HF too high compared to the measured relative interstellar abundances of these species by a factor of 1.4–4. In previous modeling calculations, the rate coefficient for the C+ + HF → CF+ + H reaction was estimated through a modified Langevin model. In the present work, rate coefficients for both spin–orbit states of the C+ ion are obtained from cross sections recently computed by Dagdigian & Kłos for this reaction through quantum statistical calculations. Their computed reaction rate coefficient is lower by a factor of ∼2 than the modified Langevin result. This lower value of the rate coefficient for the C+ + HF reaction has been employed here in new modeling of fluorine chemistry. These calculations yield an abundance of CF+ relative to that for HF in much better agreement with the astronomical observations.