Nuclear spin dependence of the reaction of ${\rm H}_3^+$H3+ with H2. II. Experimental measurements

The nuclear spin dependence of the chemical reaction \documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+ +$\end{document}H3++ H2 → H2 + \documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+ has been studied in a hollow cathode plasma cell. Multipass infrared direct absorption spectroscopy has been employed to monitor the populations of several low-energy rotational levels of ortho- and para-\documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+ (o-\documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+ and p-\documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+) in hydrogenic plasmas of varying para-H2 (p-H2) enrichment. The ratio of the rates of the proton hop (kH) and hydrogen exchange (kE) reactions α ≡ kH/kE is inferred from the observed p-\documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+ fraction as a function of p-H2 fraction using steady-state chemical models. Measurements have been performed both in uncooled (Tkin ∼ 350 K) and in liquid-nitrogen-cooled (Tkin ∼ 135 K) plasmas, marking the first time this reaction has been studied at low temperature. The value of α has been found to decrease from 1.6 ± 0.1 at 350 K to 0.5 ± 0.1 at 135 K.