Electron-Ion Recombination in Nitrogen

Abstract

Combined microwave, mass spectrometric, and optical techniques have been used to study the afterglow decay of electrons, ions, and excited atoms from microwave discharges in nitrogen-neon gas mixtures under conditions where ${\mathrm{N}}_2^{+}$ is the only significant afterglow ion, i.e., at nitrogen pressures less than 5×${10}^{-3\mathrm{}}$ Torr. Optical absorption studies show that neon-metastable atoms, an undesirable ionization source, are present in the afterglow, the concentration being inversely related to the discharge pulse length. Under conditions of no detectable metastable-atom concentration and for neon pressures in the range 15 to 30 Torr, the afterglow decay is controlled by the recombination of ${\mathrm{N}}_2^{+}$ ions and electrons, yielding a recombination coefficient $\alpha \left({{\mathrm{N}}_2}^{+}\right)=(2.9\mathrm{}\pm 0.3)\times {10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$/sec. The variation of the metastable decay rate with nitrogen pressure gives a cross section of (5.4±1.0)×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ for the de-excitation of the ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ neon-metastable state by nitrogen molecules. At higher nitrogen pressures and shorter discharge pulse lengths (25-50 μsec) the recombination controlled afterglows are dominated by ${\mathrm{N}}_4^{+}$ ions, the resulting recombination coefficient being approximately 2×${10}^{-6\mathrm{}}$ ${\mathrm{cm}}^3$/sec. All values refer to a temperature of 300°K.