\(N_2\) Active Species in Microwave Plasma and Early Afterglows at Low Gas Pressure

Abstract

The N₂ flowing microwave discharges have been investigated by emission spectroscopy in the following conditions:  N₂ at p = 5 Torr, flow rate of 0.3 slpm, t = 1.5×10^-3 s and microwave power of P = 150 W. The plasma and pink afterglows have been analysed in a setup where the end of the plasma (produced inside a 5 mm ext. dia. tube) was directly connected to a larger post-discharge tube (with a diameter of 15 mm). Under these conditions, the production of N₂⁺ ions in the plasma and in the early afterglow is done predominantly by electron collisions. After the end of the plasma up to the pink afterglow, N₂(A) and N₂(X,v>11) were the excited species at the origin of the N₂(C) and N₂⁺(B) emissions. A self-consistent theoretical model was introduced to explain the production of the N₂ active species by particle kinetics.

The intensity ratio  increased 15 times from the plasma to the pink afterglow. In this setup, the gas temperature was about 1000 K in the plasma and 700 K in the pink afterglow. In a second setup, the plasma end was at 12 cm of the post-discharge tube. The gas temperature was lower: 750 K at 2 cm and 400 K at the plasma end (3.5 cm). The effect of adding an Ar-NO_ext gas mixture (used for N-atom titration in the late afterglow inside the 15 mm diameter tube) at the discharge end was studied along the 5 mm diameter tube. It was concluded first that the destruction of N-atoms by NO_ext decreased the intensity of the N(²P) and N₂(B) radiative states. Second, the electrons at the plasma end could excite the N₂(C) and N₂⁺(B) radiative states.