BRIAN A. TOM, BRETT A. MCGUIRE, LAUREN E. MOORE, THOMAS J. WOOD, Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801; BENJAMIN J. MCCALL, Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801.

H3+ is the key precursor to ion chemistry in the interstellar medium. It has been employed as an astrophysical probe of conditions of temperature and density due to its ubiquity in a variety of environments. The distribution of ortho- and para- spin modifications of H3+ is particularly interesting in this regard. Consequently, it is important to understand the pathways through which changes to the H3+ spin distribution can occur. One possible pathway is the H3+ + H2 rightarrow H2 + H3+ reaction, which proceeds by proton hop and proton exchange and is governed by the conservation of nuclear spin. Cordonnier et al. studies this reaction at high temperature in a pulsed hollow cathode cell, but to facilitate the understanding of astronomical observations, we need lower temperature measurements. Recently, we have constructed a liquid nitrogen-cooled hollow cathode discharge source and coupled it with multipass absorption spectroscopy to measure the ortho:para ratio of H3+ in plasmas at a variety of para-H2 enrichment levels at sim160 K. Previously, we have reported\footnote[2]B.~A.~Tom, M.~B.~Wiczer, A.~A.~Mills, K.~N.~Crabtree, and B.~J.~McCall, ``Observation of nuclear spin selection rules in supersonically expanding plasmas containing H3+,'' 63^rd International Symposium on Molecular Spectroscopy (2008). experimental measurements of the branching ratio between proton hop and exchange in a hydrogenic plasma at sim80 K. Together, these experiments have allowed us to explore the temperature dependence of this branching ratio and provide valuable information for the interpretation of astronomical observations.