Recombination, Attachment, and Ambipolar Diffusion of Electrons in Photo-Ionized NO Afterglows

Abstract

Photo-ionized plasma afterglows of NO have been studied by combined microwave and mass-spectrometric techniques. Nitric-oxide-neon mixtures (5-65 mTorr NO, 2-7 Torr Ne) are contained in a 10-cm resonant cavity where they are ionized by a "single pulse" of Lyman-$\alpha$ radiation. A temporal spectrum of ions diffusing to the wall is obtained by a differentially pumped mass spectrometer and multichannel analyzer. Analysis of the electron-density decay curves obtained by microwave techniques to obtain an electron-ion recombination coefficient for N${\mathrm{O}}^{+}$ is complicated by the conversion of N${\mathrm{O}}^{+}$ to the dimer ion, ${\mathrm{}\left(\mathrm{NO}\right)\mathrm{}}_2^{+}$. At sufficiently low densities of nitric oxide the ${\mathrm{}\left(\mathrm{NO}\right)\mathrm{}}_2^{+}$ concentration becomes negligible, and the N${\mathrm{O}}^{+}$ wall current tracks the electron-density decay. From comparisons of experimental electron-density decay curves obtained under recombination-controlled conditions, with computer solutions of the electron-continuity equation, the values $\alpha \left(\mathrm{N}{\mathrm{O}}^{+}\right)=(7.4\mathrm{}\pm 0.7), (4.1\mathrm{}\pm \left\{0.3\right\}\left\{0.2\right\}), \mathrm{and} (3.1\mathrm{}\pm 0.2)\times {10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$/sec, at $T=200, 300, \mathrm{and} 450°$K, respectively, are obtained. From analysis of electron-density decay curves at higher densities of NO, where ${\mathrm{}\left(\mathrm{NO}\right)\mathrm{}}_2^{+}$ is the dominant ion, the value $\alpha {{\left(\mathrm{NO}\right)}_2}^{+}=(1.7\mathrm{}\pm 0.4)\times {10}^{-6\mathrm{}}$ ${\mathrm{cm}}^3$/sec at $T=300\mathrm{}°$K is obtained. The three-body electron-attachment and ambipolar diffusion coefficients have been measured in pure NO (0.1-5 Torr) and are found to be $K=(1.3\mathrm{}\pm 0.1)\times {10}^{-31\mathrm{}}$ ${\mathrm{cm}}^6$/sec and $D_{a}p=80\mathrm{}\pm 16$ ${\mathrm{cm}}^2$ Torr/sec, respectively, at $T=300\mathrm{}°$K.