Positive Ions in Nitric Oxide Afterglows

Abstract

Reported in this paper are results of measurements performed on a stationary nitric oxide afterglow. The nitric oxide was contained in a large gold-plated cylinder in which it was ionized by krypton resonance radiation. The ions from the afterglow were sampled through an electrically insulated orifice; it was observed that the potentials (in the mV range) applied to the orifice had a pronounced effect on the measured time constant of the afterglow decay. After the ions emerged from the sampling orifice, they were quantitatively detected by time-resolved mass spectrometry. An extensive study was made of the assumptions involved in the data-analysis model in order to establish the validity of the measurement techniques. In pure nitric oxide the only reaction observed over the pressure range investigated (10-300 m Torr) was $\mathrm{N}{\mathrm{O}}^{+}+\mathrm{NO}+\mathrm{NO}\stackrel{k}{\to }\mathrm{N}{\mathrm{O}}^{+}·\mathrm{NO}+\mathrm{NO}$. The rate constant $k$ was determined to be 5 ± 1 × ${10}^{-30\mathrm{}}$ ${\mathrm{cm}}^6$/sec. Measurements were also made of the diffusion coefficients for N${\mathrm{O}}^{+}$ and N${\mathrm{O}}^{+}$. NO in nitric oxide gas. The afterglow plasma exhibits a sharp transition from positive-ion-electron ambipolar-diffusion domination $D_{+,e}$ to positive-ion-negative-ion ambipolar-diffusion domination $D_{+,-}$. The negative ion participating in this latter diffusion process is N${\mathrm{O}}_2^{-}$. These data represent the first reported observation of such a transition. For N${\mathrm{O}}^{+}$ the experimental values are $D_{+,e}p=85\mathrm{}\pm 10$ ${\mathrm{cm}}^2$ Torr/sec and $D_{+,-}p=42\mathrm{}\pm 5$ ${\mathrm{cm}}^2$ Torr/sec. Similarly, for N${\mathrm{O}}^{+}$. NO the values are $D_{+,e}p=84\mathrm{}\pm 13$ ${\mathrm{cm}}^2$ Torr/sec and $D_{+,-}p=41\mathrm{}\pm 5$ ${\mathrm{cm}}^2$ Torr/sec.