THE LOW-TEMPERATURE NUCLEAR SPIN EQUILIBRIUM OF H3+ IN COLLISIONS WITH H2

Recent observations of H-2 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 H-2 should undergo frequent thermalizing collisions in molecular clouds. Non-thermal behavior of the fundamental H-3(+)/H-2 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 H-2 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.