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

In diffuse molecular clouds, the nuclear spin temperature of H+3 (∼30 K) is much lower than the cloud kinetic temperature ( ∼ 70 K). To understand this temperature discrepancy, we have measured the ratio of the hop to exchange pathways (α) in the H+3 + H2 → H2 + H+3 reaction (which interconverts ortho- and para-H+3) using high resolution spectroscopy of the ν2 fundamental band of H+3 in a hydrogenic plasma. We find that α decreases from 1.6 ± 0.1 at 350 K down 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 + H2 reaction is large. Our results highlight the need for further studies of the H+3 + H2 reaction as well as state-selective measurements of H+3 dissociative recombination. Key words: hydrogen, nuclear spin, spectroscopy, interstellar clouds. 1