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

The discovery of H3+ 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 H3+ in these clouds is inconsistent with the temperature derived from the excitation of H2, 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+ using the CRYRING and TSR ion storage rings. We have also performed the first measurements of the reaction H3+ + H2 → H2 + H3+ 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-H3+. Finally, we have constructed a steady-state chemical model for diffuse clouds, which takes into account the spindependence of the formation of H3+, its electron recombination, and its reaction with H2. We find that the ortho:para ratio of H3+ in diffuse clouds is likely governed by a competition between dissociative recombination and thermalization by reactive collisions. [ABSTRACT FROM AUTHOR]