Temporal evolution of the relative vibrational population of N2 (C3∏u) and optical emission spectra of atmospheric pressure plasma jets in He mixtures

In this study, spatial-temporal resolved optical emission spectroscopy and electrical characteristics are employed to study the dynamic evolution of molecules, vibrational distributions, reactive species, and streamer head speed in the generation and propagation of atmospheric pressure plasma jets (APPJs). The images of discharge, waveforms of pulse peak voltage and discharge current, and spatial-temporal emission spectra of N2 (C3Πu → B3Πg, 380.5 nm), (B2 → X2, 391.4 nm) and He (3s3S → 2p 3P, 706.5 nm) are recorded, the relative vibration population of N2 (C3Πu) and the key dynamic process of each discharge pulse are discussed. The effects of pulse peak voltage on emission intensity, vibration population of N2 (C3Πu) and APPJ speed are also investigated. The results show that the streamer head speed is about 105 m s−1 under the pulse peak voltage of 5–9 kV, and both the streamer head speed and emission intensities increase with pulse peak voltage. The emission intensities of N2 (C3Πu → B3Πg, 380.5 nm) rise for about 10 ns but fall for several tens of nanoseconds. During the plasma generation process, the direct electron impact process is dominant in generating electronic, vibrationally and rotationally excited N2. Several tens of nanoseconds after the generation, spontaneous emission, quenched by N2 and O2 dominate the decay process. While the step excitation and Penning ionization extend the duration time of emission intensity. It is also found that the ratio of N2 (C, υ = 1)/N2 (C, υ = 0) is at high level within . The evolution of the ratio is dominated by the direct electron impact in the initial time, and is then possibly influenced by the vibrational relaxation process and downward vibrational–vibrational energy transmission process.