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

Abstract Rotational temperature measurements of N 2 (B 3 Π g ), N 2 (C 3 Π u ) and N 2 + ( B 2 Σ 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 Σ u + ) and N + . Kinetic and QSS simulations are performed to support this assumption and a rate constant for this reaction is proposed.