The ortho: Para ratio of H₃⁺ 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 (a) in the H₃ ⁺ + H₂→H₂ + H₃⁺ reaction (which interconverts ortho- and para-H₃⁺ ) using high-resolution spectroscopy of the ν2 fundamental band of H₃ ⁺ in a hydrogenic plasma. We find that a decreases from 1.6 ±H₃⁺0.1 at 350K to its statistical value of 0.5 ±H₃⁺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 orthoand para-H₃⁺ are equal and the identity branching fraction for the H₃⁺ + H₂ reaction is large. Our results highlight the need for further studies of the H₃⁺ + H2 reaction as well as state-selective measurements of H₃⁺ dissociative recombination.