Recent observations of H2 and H+ 3 in diffuse interstellar sightlines revealed a difference in the nuclear spin excitation temperatures of the two species. This discrepancy comes as a surprise, as H+ 3 and H2 should undergo frequent thermalizing collisions in molecular clouds. Non-thermal behavior of the fundamental H+ 3/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 H+ 3 in collisions with H2 molecules in a temperature-variable radiofrequency ion trap between 45 and 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.
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