THE LOW-TEMPERATURE NUCLEAR SPIN EQUILIBRIUM OF H$_3^+$ IN COLLISIONS WITH H$_2$
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Date
2013
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Publisher
Ohio State University
Abstract
Observations of H$_3^+$ in diffuse molecular clouds have revealed that the ratio of its \textit{ortho} and \textit{para} nuclear spin modifications are not in thermodynamic equilibrium with the environment. This discrepancy could be explained if the reaction H$_3^+$ + H$_2$ $\to$ H$_2$ + H$_3^+$, which interconverts the nuclear spin modifications of H$_3^+$, has a nonthermal outcome at low temperatures, possibly arising from nuclear spin selection rules on systems of identical fermions. While the nuclear spin dependence of this reaction has previously been investigated experimentally, the prior measurements were limited to temperatures above $\sim$130 K, well above the 50-70 K typical of diffuse molecular clouds. To investigate whether the outcome of the H$_3^+$ + H$_2$ reaction is nonthermal, H$_3^+$ ions were allowed to interact with H$_2$ in the temperature-controlled environment of a 22-pole radiofrequency ion trap, and the relative abundances of \textit{ortho}- and \textit{para}-H$_3^+$ at steady state were measured using action spectroscopy. By carefully controlling the \textit{ortho}:\textit{para} ratio of the H$_2$ samples in conjunction with the ion trap temperature, the outcome of the reaction was observed to be close to thermodynamic equilibrium over the temperature range of 45-100 K. Thus, the nonequilibrium \textit{ortho}:\textit{para} ratio of H$_3^+$ observed in diffuse molecular clouds does not arise from a nonthermal outcome of the H$_3^+$ + H$_2$ reaction at low temperature. This implies that the origin of the discrepancy lies in the respective formation and destruction mechanisms of H$_3^+$.
Description
Author Institution: Max-Planck-Institut fur Kernphysik, 69117 Heidelberg, Germany; Department of Chemistry, University of Illinois, Urbana, IL, 61801; I. Physikalisches Institut, Universitat zu Koln, 50937 Koln, Germany