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.
@article{arxiv.1209.3041,
title = {The Low-Temperature Nuclear Spin Equilibrium of H3+ in Collisions with H2},
author = {F. Grussie and M. H. Berg and K. N. Crabtree and S. Gaertner and B. J. McCall and S. Schlemmer and A. Wolf and H. Kreckel},
journal= {arXiv preprint arXiv:1209.3041},
year = {2015}
}