Excitation of O(1S) auroral green line from ambient air in a microwave-induced plasma jet at atmospheric pressure

Strong emission from the auroral green line O(1S) → O(1D) at 557.7 nm was observed in the afterglow of an atmospheric-pressure 2.45 GHz microwave-induced plasma jet. The device used Ar as the working gas for the plasma, and a surrounding N2 gas flow to stabilize the discharge. The characteristic line-shape of the peak at 557.7 nm indicated that the emission was from the O(1S)N2 excimer. Vibrational temperatures of NO and N2, estimated based on the emission spectra of the afterglow, were compared against the intensity of the green line and the concentration of N2 gas flowing in the system. The data points yielding the most intense green lines fall at vibrational temperatures of around 2000 K for both NO and N2. It was observed that the amount of N2 gas flowing in the system positively contributed to the intensity of the green line. Using fluid simulation, the O2 concentration in the afterglow was estimated to be in the range of 1-5.5%.