The ortho : para ratio of H 3 + in laboratory and astrophysical plasmas

In diffuse molecular clouds, the nuclear spin temperature of H(3)(+) (approx. 30 K) is much lower than the cloud kinetic temperature (approx. 70 K). To understand this temperature discrepancy, we have measured the ratio of the hop to exchange pathways (α) in the H(3)(+) + H(2) --> H(2) + H(3)(+) reaction (which interconverts ortho- and para-H(3)(+)) using high-resolution spectroscopy of the ν(2) fundamental band of H(3)(+) in a hydrogenic plasma. We find that α decreases from 1.6±0.1 at 350 K to its statistical value of 0.5±0.1 at 135 K. We use this result to model the steady-state chemistry of diffuse molecular clouds, finding good agreement with astronomical data provided the dissociative recombination rates of ortho- and para-H(3)(+) are equal and the identity branching fraction for the H(3)(+) + H(2) reaction is large. Our results highlight the need for further studies of the H(3)(+) + H(2) reaction as well as state-selective measurements of H(3)(+) dissociative recombination.