The Low-Temperature Nuclear Spin Equilibrium of H3+ in Collisions with   H2
  Recent observations of H2 and H3+ in diffuse interstellar sightlines revealed a difference in the nuclear spin excitation temperatures of the two species. This discrepancy comes as a surprise, as H3+ and H2 should undergo frequent thermalizing collisions in molecular clouds. Non-thermal behavior of the fundamental H3+ / H2 collision system at low temperatures was considered as a possible cause for the observed irregular populations. Here, we present measurements of the steady-state ortho/para ratio of H3+ in collisions with H2 molecules in a temperature-variable radiofrequency ion trap between 45-100 K. The experimental results are close to the expected thermal outcome and they agree very well with a previous micro-canonical model. We briefly discuss the implications of the experimental results for the chemistry of the diffuse interstellar medium. 
F. Grussie, M. H. Berg, K. N. Crabtree, S. Gaertner, B. J. McCall, S.   Schlemmer, A. Wolf, and H. Kreckel
astro-ph.GA physics.chem-ph