Utilizing different ratios of para to ortho ${\mathrm{H}}_2$ in normal and para enriched hydrogen, we varied the population of para-$\mathrm{H}_3{}^{+}$ in an $\mathrm{H}_3{}^{+}$ dominated plasma at 77 K. Absorption spectroscopy was used to measure the densities of the two lowest rotational states of $\mathrm{H}_3{}^{+}$. Monitoring plasma decays at different populations of para-$\mathrm{H}_3{}^{+}$ allowed us to determine the rate coefficients for binary recombination of $\mathrm{H}_3{}^{+}$ and ortho-$\mathrm{H}_3{}^{+}$ ions: ${}^p\alpha _{\mathrm{bin}}(77\mathrm{K})=(1.9\pm 0.4)\times {10}^{-7}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ and ${}^o\alpha _{\mathrm{bin}}(77\mathrm{K})=(0.2\pm 0.2)\times {10}^{-7}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$.

• Received 13 September 2010

DOI:https://doi.org/10.1103/PhysRevLett.106.203201