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 N + 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 Tort by means of mass spectrometer techniques. During the early part of the afterglow period the time dependence of N + is controlled by ambipolar diffusion loss towards the plasma container walls. The product of the ambipolar diffusion coefficient D a and the reduced pressure Po was DaPo = 900__ 50 cm 2 Torr/sec. The production of N + by collisions between metastable nitrogen molecules determines the temporal behavior of the N + 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 x I0 -is cm 3 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 N + is produced during the plasma decay period by the process N2 (a' 1 v~-) + N2 (a' 1 ~v~-) ._. N + (X 2 v+) + Nz (X) + e.

Afterglows

“…However, as the previous sections show, the lifetimes of these states in the presence of K atoms and at a total pressure of more than 100 Pa are much shorter than the observed duration of the pink afterglow. Therefore, although energy pooling by such molecules can yield energy-rich N, molecules and ions (Lund andOskam 1969, DeMonchy and, they cannot be considered as the primary source.…”