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

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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 ${\mathrm{NO}}_{2}^{\ensuremath{-}}$, 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\ifmmode^\circ\else\textdegree\fi{}K, is ${4.6}_{\ensuremath{-}1.3}^{+0.5}$\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. Similar measurements made at temperatures of 196 and 358\ifmmode^\circ\else\textdegree\fi{}K yield the values ${10}_{\ensuremath{-}4}^{+2}$\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ and ${3.5}_{\ensuremath{-}0.5}^{+0.2}$\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$, respectively. The results of this study together with other laboratory measurements suggest an approximate ${T}^{\ensuremath{-}1.2}$ dependence of the recombination coefficient.