PRODUCTION AND LOSS OF H+/2 IONS DURING DECAY PERIOD OF PLASMAS PRODUCED IN HELIUM-NITROGEN MIXTURES

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 coefficient Da and the reduced pressure p0 was Dap0=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 Σu-)+N2 (a′1σu-→N2+ (X2Σu+)+N2(X)+e. © 1969 Springer-Verlag.