Vibrational and rotational temperatures of NO A 2Σ+ metastable state in N2-O2 mixture microwave discharge

Summary form only given. The spectra are observed in microwave discharge plasma experiments of N₂-O₂ gas mixture discharge plasma, particularly for γ-band of NO molecule in UV region for examination of excited-state population of NO molecule. The plasma is generated in a quartz tube with its inner diameter 26 mm inserted in a rectangular waveguide and its discharge pressure about several Torr. From the observed spectroscopic results, we can find that both NO and N₂ molecules experience a cooling down process both on vibrational and rotational temperatures as the plasma flows to the downstream direction. It is found that NO molecule has always higher rotational temperature than N₂ for the present discharge condition. Meanwhile, we found that in this nonequilibrium plasma, both NO and N₂ molecules always get higher energy for vibrational motion than for rotational motion. As a next experiment, we measured the dependence of the temperatures on the N₂-O₂ gas-mixture ratio. Consequently, we found a remarkable change in the vibrational temperature of NO A state. As O₂ molar ratio of the mixture increases, the NO experiences a monotonic increase in its vibrational temperature of the A state¹. This is because that the NO A ²Σ⁺ metastable state is excited from two main paths: (1) N₂(A ³Σ_u⁺) + NO(X ²Π) → N₂(X ¹ Σ_g⁺) + NO(A ²Σ⁺), (2) NO(X ²Π) + e^- → NO(A ²Σ⁺) + e^-. When N₂ is the majority of the discharge species, reaction (1) is considered to dominate the excitation process of NO A ²Σ⁺, while for the O₂-dominant discharge, reaction (2) should dominate. T- erefore, under our discharge conditions, vibration-vibration energy transition of the reaction (1) results in a strong vibrational relaxation of NO(A ²Σ⁺) state molecules when N₂ is the majority. In conclusion, the admixture of N₂ gas can lead to the reduction of average vibrational temperature of NO A state significantly. We discuss these excitation kinetics with a modified numerical code as a global model in a cylindrical tube, where EEDF and N₂-VDF are treated as in a state of non-equilibrium together with several molecular excited species².