H$_{2}$-Assisted Ternary Recombination of H$_{3}^{+}$ with Electrons at 300 K

Afterglow measurements in ionized He/Ar/H$_{2}$ gas mixtures at 300 K show that the recombination of H$_{3}^{+}$ ion with electrons is very strongly enhanced in the presence of molecular hydrogen. In the experiments the decay of H$_{3}^{+}$ ions was measured by near-infrared (NIR) absorption spectroscopy (SA-CRDS).\footnote{P. Macko et al, \textit{Int. J. Mass Spectrom.}~\textbf{233}, 299 (2004).} Rather surprisingly, the H$_{2}$-assisted three-body recombination coefficient ($K_{\mathrm{H2}} = $ (8.7 $+$/- 1.5) $\times$ 10$^{-23}$ cm$^{6}$s$^{-1})$ exceeds by more than two orders of magnitude the corresponding He-assisted coefficient ($K_{\mathrm{He}} = $ (3.3 $+$/- 0.7) $\times$ 10$^{-25}$ cm$^{6}$s$^{-1})$ that we measured earlier.\footnote{R. Johnsen et al, \textit{J. Phys. Chem. A }\textbf{11}7, 9477 (2013).} Formation of faster recombining H$_{5}^{+}$ cluster ions does not play a significant role at temperature near 300 K. The ternary processes are found to saturate at high He and H$_{2}$ densities, suggesting that recombination proceeds by a two-step process, electron capture (with a rate coefficient $\alpha_{\mathrm{F}} = $ (1.5 $+$/- 0.1) $\times$ 10$^{-7}$ cm$^{3}$s$^{-1})$ into a long-lived Rydberg state with an excited core, followed by collisional stabilization. While these findings provide a plausible explanation for some of the discrepancies between earlier afterglow measurements of H$_{3}^{+}$ recombination, the exact nature of these long-lived complexes, and their collisional interactions remain to be elucidated.