The Low-temperature nuclear spin equilibrium of H⁺₃ in collisions with H₂

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 100K. 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.