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

The recombination of 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 NO⁺ ions. Negative ions, predominantly NO₂^-, 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 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+0.5×{}10^-7 cm³ sec^-1. Similar measurements made at temperatures of 196 and 358^∘{}K yield the values 10_-4⁺²×{}10^-7 and 3.5_-0.5+0.2×{}10^-7 cm³ sec^-1, respectively. The results of this study together with other laboratory measurements suggest an approximate T^-1.2 dependence of the recombination coefficient.