THE LOW-TEMPERATURE NUCLEAR SPIN EQUILIBRIUM OF H super(+) sub(3) IN COLLISIONS WITH H sub(2)

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