Spectroscopic Examination of Vibrational and Rotational Properties of NO A$^{2}\Sigma^{+}$ Metastable State from NO $\gamma $-Band Spectra in N$_{2}$-O$_{2}$ Mixture Microwave Discharge

The spectra are observed in our microwave discharge plasma experiments. N$_{2}$-O$_{2}$ mixture plasma is generated by using a rectangular waveguide. We measured the spectra at 0, 60, 100 and 140 mm with the discharge pressure several Torrs. From these results, we can find that both NO and N$_{2}$ molecules experience a cooling down process both on vibrational and rotational temperatures as the plasma flows to the downstream direction. And NO molecule has always a higher rotational temperature than N$_{2}$. Meanwhile, we can see that in this nonequilibrium plasma, both NO and N$_{2}$ molecules tend to get higher energy for vibrational motion than for rotational motion. We also change the gas partial pressure rate, when O$_{2}$ molar ratio of the mixture increases, the NO experiences an increasing vibrational temperature. This is because that the NO A$^{2}\Sigma ^{+}$ metastable state is excited from two main paths: N$_{2}$(A $^{3}\Sigma _{\mathrm{u}}^{+})+$NO(X $^{2}\Pi ) \to $N$_{2}$(X $^{1}\Sigma _{\mathrm{g}}^{+})+$NO(A $^{2}\Sigma ^{+})$, (1) NO(X $^{2}\Pi )+$e$^{-} \to $NO(A $^{2}\Sigma^{+})+$e$^{-}$ (2) When O$_{2}$ or N$_{2}$ is the majority of the discharge species, reaction (2) or (1) dominates the excitation process of NO A$^{2}\Sigma^{+}$, respectively. Therefore, under our plasma conditions, vibration-vibration energy transition of the reaction (1) results in a strong vibrational relaxation of NO A$^{2}\Sigma ^{+}$ state molecules when N$_{2}$ is the majority in the discharge gas. In conclusion, the admixture of N$_{2}$ gas can lead to the reduction of average vibrational temperature significantly.