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-$\ensuremath{\alpha}$ 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, ${(\mathrm{NO})}_{2}^{+}$. At sufficiently low densities of nitric oxide the ${(\mathrm{NO})}_{2}^{+}$ 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 $\ensuremath{\alpha}(\mathrm{N}{\mathrm{O}}^{+})=(7.4\ifmmode\pm\else\textpm\fi{}0.7), (4.1\ifmmode\pm\else\textpm\fi{}{0.3}{0.2}), \mathrm{and} (3.1\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ ${\mathrm{cm}}^{3}$/sec, at $T=200, 300, \mathrm{and} 450\ifmmode^\circ\else\textdegree\fi{}$K, respectively, are obtained. From analysis of electron-density decay curves at higher densities of NO, where ${(\mathrm{NO})}_{2}^{+}$ is the dominant ion, the value $\ensuremath{\alpha}{{(\mathrm{NO})}_{2}}^{+}=(1.7\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ ${\mathrm{cm}}^{3}$/sec at $T=300\ifmmode^\circ\else\textdegree\fi{}$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 $K=(1.3\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}31}$ ${\mathrm{cm}}^{6}$/sec and ${D}_{a}p=80\ifmmode\pm\else\textpm\fi{}16$ ${\mathrm{cm}}^{2}$ Torr/sec, respectively, at $T=300\ifmmode^\circ\else\textdegree\fi{}$K.