Electron-Ion Recombination in Nitric Oxide in the Temperature Range 196 to 358°K

Abstract

The recombination of ${\mathrm{NO}}^{+}$ ions with electrons was studied in photoionized nitric-oxide-neon gas mixtures. A pulsed hydrogen lamp with a lithium fluoride window produced the ionizing ultraviolet radiation, which was incapable of exciting or ionizing Ne. Following an ionizing pulse, the decay of electron density was measured by a microwave-cavity reflection technique. Low NO pressures, 0.5 to 80 mTorr, were used to reduce negative-ion formation, and high Ne pressures, up to 132 Torr, were used to reduce ambipolar diffusion. Highly purified gases and ultrahigh-vacuum techniques were employed. Mass-spectrometer identification of the positive ion in NO-Ne mixtures indicated the presence of only ${\mathrm{NO}}^{+}$ ions. Negative ions, predominantly ${\mathrm{NO}}_2^{-}$, were observed to increase in number rapidly as NO pressure increased above 3 mTorr, and the uncorrected electron-recombination coefficient, computed from plots of reciprocal frequency shift versus time, increased as NO pressure increased, very likely owing to the influence of negative ions. Correction for negative-ion and diffusion effects was made by use of a computer-aided analysis. The ${\mathrm{NO}}^{+}$-electron recombination coefficient, measured at NO pressures of 0.5 and 1.0 mTorr under conditions of small negativeion influence and at a gas temperature of 298°K, is ${4.6}_{-1.3\mathrm{}}^{+0.5\mathrm{}}$×${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$. Similar measurements made at temperatures of 196 and 358°K yield the values ${10}_{-4\mathrm{}}^{+2\mathrm{}}$×${10}^{-7\mathrm{}}$ and ${3.5}_{-0.5\mathrm{}}^{+0.2\mathrm{}}$×${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$, respectively. The results of this study together with other laboratory measurements suggest an approximate $T^{-1.2\mathrm{}}$ dependence of the recombination coefficient.