Measurements of the Recombination of Electrons with ${\mathrm{H}}_3{\mathrm{O}}^{+}·{\left({\mathrm{H}}_2\mathrm{O}\right)}_n$-Series Ions

Recombination coefficients of electrons with ions of the hydrated hydronium-ion series ${\mathrm{H}}_3{\mathrm{O}}^{+}·{\left({\mathrm{H}}_2\mathrm{O}\right)}_n$, where $n=0,1,2,3,4,5,6\mathrm{}$, have been determined with a microwave after-glow/mass-spectrometer apparatus. Afterglow measurements of electron density decays in helium-water-vapor mixtures are correlated with the decay of mass-identified ion currents to the wall of the microwave cavity. By varying the temperature of the gas and the partial pressure of the water vapor in the mixture, different groups of hydronium-series ions are made to dominate the afterglow. The measured recombination coefficients, in units of ${10}^{-6\mathrm{}}$ ${\mathrm{cm}}^3$/sec, are given. The implications of these results concerning ionization levels in the $D$ region of the ionosphere are discussed.