Production and loss of N₂ ⁺ ions during the decay period of plasmas produced in helium-nitrogen mixtures

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.