Ambipolar Diffusion and Electron Attachment in Nitric Oxide in the Temperature Range 196 to 358°K

Ambipolar diffusion and electron attachment rates were measured in nitric oxide at pressures from 0.01 to 16 Torr and at temperatures from 196 to 358°K. The gas was photoionized by single pulses of ultraviolet radiation, at and near Lyman-$\alpha$ from a hydrogen lamp. Pulse lengths ranged from 10 to 300 μsec and ionization levels were kept low to reduce electron-ion recombination. Rates for the latter process are reported in a separate paper. Electron loss rates were measured by a microwave-cavity method used to record the decay following a single ionizing pulse. Ion identification was made by a mass spectrometer which sampled the ions diffusing through a small hole in the wall of the microwave cavity. Ultrahigh-vacuum techniques were used in gas purification and in production and measurement of ionization. The diffusion results are to some extent consistent with the theory of diffusion of electrons and a single positive ion species in the presence of negative ions. The mass-spectrometer observations indicate that ${\mathrm{NO}}^{+}$ is the dominant positive ion in photoionized NO at all pressures. The measured mobility of ${\mathrm{NO}}^{+}$ for 298°K, referred to 0°C and 760 Torr, is 1.9±0.2 ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{V}}^{-1\mathrm{}}$, corresponding to a $D_{a}p$ of 83±8 ${\mathrm{cm}}^2$ Torr ${\sec }^{-1\mathrm{}}$. The mobility decreases slowly as the gas temperature is raised from 196 to 358°K. At gas densities near ${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$, the attachment results are characteristic of a three-body process, but at higher densities there is an anomalous rise in the attachemnt coefficient. The apparent three-body attachment coefficients for densities near ${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$ are 6.8±0.7×${10}^{-31\mathrm{}}$ ${\mathrm{cm}}^6$ ${\sec }^{-1\mathrm{}}$ at 196°K, 2.2±0.2×${10}^{-31\mathrm{}}$ at 298°K, and 1.1±0.1×${10}^{-31\mathrm{}}$ at 358°K and exhibit approximately at $T^{-3\mathrm{}}$ temperature dependence. The principal negative ion observed with the mass spectrometer was ${\mathrm{NO}}_2^{-}$, and no ${\mathrm{NO}}^{-}$ or ${\mathrm{}\left(\mathrm{NO}\right)\mathrm{}}_2^{-}$ ions were detected. Direct attachment to ${\mathrm{NO}}_2$, present as a minute impurity, seems unlikely; the ${\mathrm{NO}}_2^{-}$ ions may be formed by rapid charge transfer from a primary negative ion or by an attachment reaction involving rearrangement.