Measurements of Recombination of Electrons withH3+andH5+Ions

The electron-ion recombination coefficients for $\mathrm{H}_3^{}{}_{}{}^{+}$ and $\mathrm{H}_5^{}{}_{}{}^{+}$ ions have been determined by means of a microwave afterglow/mass-spectrometer apparatus. Measurements of electron density decays in helium-hydrogen mixtures are correlated with the decay of mass-identified ion currents to the wall of the microwave cavity. At low partial pressures of hydrogen in the mixture, the ion $\mathrm{H}_3^{}{}_{}{}^{+}$ dominates the ion composition and the ion wall current "tracks" the electron density decay curves. From recombination controlled electron density decay curves, the values $\alpha \left({{\mathrm{H}}_3}^{+}\right)=(2.9\mathrm{}\pm 0.3),(2.3\pm 0.3),\mathrm{and} (2.0\mathrm{}\pm 0.2)\times {10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$/sec are obtained at 205, 300, and 450 K, respectively. At higher partial pressures of hydrogen and low temperatures, where $\mathrm{H}_5^{}{}_{}{}^{+}$ is the dominant ion, the value $\alpha \left({{\mathrm{H}}_5}^{+}\right)=(3.6\mathrm{}\pm 1.0)\times {10}^{-6\mathrm{}}$ ${\mathrm{cm}}^3$/sec is obtained at 205 K. The implications of these results concerning ionization levels in the atmospheres of the outer planets and in the intersteller medium are discussed.