Knowledge Bank

H$\mathrm{3+}$ 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 H$\mathrm{3+}$ is particularly interesting in this regard. Consequently, it is important to understand the pathways through which changes to the H$\mathrm{3+}$ spin distribution can occur. One possible pathway is the H$\mathrm{3+}$ + H$2\to$ H$2$ + H$\mathrm{3+}$ reaction, which proceeds by proton hop and proton exchange and is governed by the conservation of nuclear spin. Cordonnier et al., 3181 (2000)} 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 H$\mathrm{3+}$ in plasmas at a variety of para-H$2$ enrichment levels at $\sim$160\; K.\; Previously,\; we\; have\; reported\}$ International Symposium on Molecular Spectroscopy (2008).} experimental measurements of the branching ratio between proton hop and exchange in a hydrogenic plasma at $\sim$80 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.