FLORIAN GRUSSIE, MAX H. BERG, ANDREAS WOLF AND HOLGER KRECKEL, Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany; KYLE N. CRABTREE AND BENJAMIN J. MCCALL, Department of Chemistry, University of Illinois, Urbana, IL, 61801; SABRINA GÄRTNER AND STEPHAN SCHLEMMER, I. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany.

Observations of H3+ in diffuse molecular clouds have revealed that the ratio of its ortho and para nuclear spin modifications are not in thermodynamic equilibrium with the environment. This discrepancy could be explained if the reaction H3+ + H2 \to H2 + H3+, which interconverts the nuclear spin modifications of H3+, 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 sim130 K, well above the 50-70 K typical of diffuse molecular clouds.

To investigate whether the outcome of the H3+ + H2 reaction is nonthermal, H3+ ions were allowed to interact with H2 in the temperature-controlled environment of a 22-pole radiofrequency ion trap, and the relative abundances of ortho - and para -H3+ at steady state were measured using action spectroscopy. By carefully controlling the ortho : para ratio of the H2 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 ortho : para ratio of H3+ observed in diffuse molecular clouds does not arise from a nonthermal outcome of the H3+ + H2 reaction at low temperature. This implies that the origin of the discrepancy lies in the respective formation and destruction mechanisms of H3+.