The Ortho:para Ratio of H3+ in Laboratory and Astrophysical Plasmas

The discovery of H-3(+) in the diffuse interstellar medium has dramatically changed our view of the cosmic-ray ionization rate in diffuse molecular clouds. However, another surprise has been that the ortho:para ratio of H-3(+) in these clouds is inconsistent with the temperature derived from the excitation of H-2 the dominant species in these clouds. In an effort to understand this discrepancy, we have embarked on an experimental program to measure the nuclear spin dependence of the dissociative electron recombination rate of H-3(+) using the CRYRING and TSR ion storage rings. We have also performed the first measurements of the reaction H-3(+) + H-2 -> H-2 + H-3(+) below room temperature. This reaction is likely the most common bimolecular reaction in the universe, and plays an important role in interconverting ortho- and para-H-3(+). Finally, we have constructed a steady-state chemical model for diffuse clouds, which takes into account the spindependence of the formation of H-3(+), its electron recombination, and its reaction with H-2. We find that the ortho:para ratio of H-3(+) in diffuse clouds is likely governed by a competition between dissociative recombination and thermalization by reactive collisions.