Investigation of nitrogen fixation in low-pressure microwave plasma via rotational-vibrational NO and N-2 kinetics

A pulsed microwave surfaguide-type discharge used for nitrogen fixation in N-2-O-2 gas mixtures is characterized by optical emission spectroscopy. Results show that both rotational and vibrational temperatures are elevated in the active zone near the waveguide, decaying along the discharge tube in both upstream and downstream. The characteristic length of optical emission from NO( A- X) transition gets contracted when pressure increases, specifically at P >= 2 Torr. The degree of vibrational non-equilibrium (defined as the ratio between vibrational and rotational temperatures) is decreased by a factor of two when pressure changes from 0.6 to 10 Torr. Non-equilibrium likely disappears as the discharge pressure rises, resulting in a gas temperature elevation. A correlation between gas residence time, pulse duration, and characteristic times for different energy transfer channels is discussed. The rotational-vibrational dynamics differs for NO and N-2 during the pulse. Both species lose vibrational excitation at the beginning of the pulse, whereas N-2 gets re-excited again during the second half of the pulse, which may occur as a result of an efficient pumping-up effect through the vibrational-vibrational energy transfer. At the same time, vibrational relaxation of NO takes place primarily due to a strong vibrational-translational exchange via NO-O-2 and NO-O collisions.