Characterization of the Active Species in the Afterglow of a Nitrogen and Helium Atmospheric-Pressure Plasma

The concentrations of the neutral active species in the afterglow of a nitrogen and helium atmospheric-pressure plasma have been determined by optical emission and absorption spectroscopy and by numerical modeling. For operation with 10 Torr N2 and 750 Torr He, at 15.5 W/cm3 rf power, 30.4 L/min flow rate, and a neutral temperature of 50°C, the plasma produced 4.8×1015 cm−3 of ground state nitrogen atoms, N(4S), 2.1×1013 cm−3 of N2(A3Σu), 1.2×1012 cm−3 of N2(B3Πg), and 3.2×109 cm−3 of N2(C3Πu). The concentration of nitrogen atoms and metastable state nitrogen molecules, N2(A), increased gradually with the rf power and the nitrogen partial pressure. Both the model and experiments indicate that ground-state nitrogen atoms are the dominant active species in the afterglow beyond 2.0 ms.