Measurements and analysis of rotational temperatures obtained with Raman and optical emission spectroscopy in a nonequilibrium nitrogen plasma

Rotational temperature measurements of N-2(B-3 Pi(g)), N-2(C-3 Pi(u)) and N-2(+)(B-2 Sigma(+)(u)) obtained by optical emission spectroscopy (OES) are compared with previous gas temperature measurements obtained with Raman spectroscopy in a nonequilibrium recombining nitrogen plasma at atmospheric pressure. The plasma is produced in a state of thermal and chemical equilibrium at about 7000 K using an inductively coupled plasma torch. It is then forced out of chemical equilibrium by imposing rapid cooling through a water-cooled tube. The gas temperature measured by Raman spectroscopy is about 3200 K at the exit of the tube. The rotational temperatures of N-2(B) and N-2(C) measured with OES are found to be in good agreement with the gas temperature, but the rotational temperature of N-2(+)(B) is found to be significantly above the gas translational temperature. We propose that this higher rotational temperature is the result of N-2(+)(B) being formed by the charge exchange reaction between N-2(A(3)Sigma(+)(u)) and N+. Kinetic and QSS simulations are performed to support this assumption and a rate constant for this reaction is proposed.