  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.
