The ortho: Para ratio of H3+ in laboratory and astrophysical plasmas

Kyle N. Crabtree, Benjamin J. McCall

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish (US)
Pages (from-to)5055-5065
Number of pages11
JournalPhilosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
Issue number1978
StatePublished - Nov 13 2012


  • Hydrogen
  • Interstellar clouds
  • Nuclear spin
  • Spectroscopy

ASJC Scopus subject areas

  • General Mathematics
  • General Engineering
  • General Physics and Astronomy


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