Production and loss of N2 + ions during the decay period of plasmas produced in helium-nitrogen mixtures |
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Authors: | A. R. DeMonchy H. J. Oskam |
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Affiliation: | 1. Department of Electrical Engineering, University of Minnesota, Minneapolis, USA
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Abstract: | The time dependence of the number density of N2 + ions during the decay period of plasmas produced in helium containing 0.05, 0.17 and 0.5 percent nitrogen was studied in the pressure range from about 0.3 to 7 Torr by means of mass spectrometer techniques. During the early part of the afterglow period the time dependence of N2 + is controlled by ambipolar diffusion loss towards the plasma container walls. The product of the ambipolar diffusion coefficientD a and the reduced pressurep 0 wasD ap0=900± 50 cm2 Torr/sec. The production of N2 + by collisions between metastable nitrogen molecules determines the temporal behavior of the N2 + density during the late afterglow for extremely pure discharge conditions. From the data it follows that the metastable molecules involved are de-excited by collisions with ground state helium atoms with a rate constant of 3.4 × 10?15 cm3 sec?1, while the radiative lifetime of these metastable molecules is at least 20 msec. The surface catalytic efficiency for de-excitation upon striking the molybdenum covered plasma container walls was estimated to be smaller than 10?3. Energy and radiative lifetime requirements suggest that N2 + is produced during the plasma decay period by the process N2 (a′1 Σ u ? )+N2 (a′1σ u ? →N2 + (X2Σ u + )+N2(X)+e. |
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