Negative-Ion Reactions in NO-H2O Mixtures

Abstract

A stationary afterglow system has been utilized to determine rate constants for thermalenergy negative-ion-molecule reactions in photoionized NO-${\mathrm{H}}_2$O mixtures. When the decay of the plasma is controlled by ambipolar diffusion of positive and negative ions, quantitative determination of rate constants is shown to be feasible. The plasma transition from electron-positive-ion ambipolar diffusive domination of the transport loss processes to domination by positive-ion-negative-ion ambipolar diffusion is explained by a model which includes the effects of negative-ion trapping. Prominant negative ions in the afterglow include N${\mathrm{O}}_2^{-}$, its hydrates, and clusters involving HN${\mathrm{O}}_2$. Reaction-rate constants for the processes N${\mathrm{O}}_2^{-}$+${\mathrm{H}}_2$O+NO → N${\mathrm{O}}_2^{-}$ · ${\mathrm{H}}_2$O + NO and ${\mathrm{Cl}}^{-}$ + ${\mathrm{H}}_2$O + N${\mathrm{O}}^{\to }$ ${\mathrm{Cl}}^{-}$ · ${\mathrm{H}}_2$O + NO are found to be 1.3±0.3×${10}^{-28\mathrm{}}$ ${\mathrm{cm}}^6$/sec and 3.4±1.3 × ${10}^{-29\mathrm{}}$ ${\mathrm{cm}}^6$/sec at 293 K, respectively. Steady glows in NO-${\mathrm{H}}_2$O-${\mathrm{O}}_2$ mixtures revealed that N${\mathrm{O}}_2^{-}$ and the impurity HC${\mathrm{O}}_3^{-}$ also formed multiple hydrates and clustered with HN${\mathrm{O}}_2$. These results indicate that the terminal negative ions in the D-region of the ionosphere will likely be hydrated.