## 3 Results and Discussions
The main reaction for the production of excited nitrogen molecules could be related to the collisions of nitrogen molecules with electrons [4,6,15]:  
Figure 2:1  
Figure 2:2  
Figure 2:3  
Figure 2:4  
After the collision of electrons with nitrogen molecule, the excited \(\mathrm{N_{2}}\) molecule is formed (a transition from the ground state (\(\mathrm{X^{1}\Sigma^{+}}_{g}\)) to the excited state (\(\mathrm{A^{3}\Sigma^{+}}_{u}\)) occurs) (Equation (4)). Its transition from the excited state of \(\mathrm{A^{3}\Sigma^{+}}_{u}\) to the \(\mathrm{C^{3}\Pi_{u}}\) state is generated by nitrogen molecules collisions with electrons (Equation (5)). Meanwhile, the formation of nitrogen molecules with excited states of (\(\mathrm{B^{3}\Pi_{g}}\)) is via the reactions (6) and (7). The low-intensity peak at 306.9 nm in the emission spectrum is attributed to the OH radical (Figure 7). It was pointed out that the emission lines attributed to the OH and NO species are usually obtained in air plasma in UV region [11,39]. The appearance of the OH radical is due to the existence of water vapor in the ambient air [39]. Thus, the air plasma was additionally measured by a flame-emission spectrometer. The emission spectra obtained at the 90 V and 250 V voltages are presented in Figure 8a,b. Their main peaks were related to exited nitrogen molecules and molecular nitrogen ions (Figure 8a). However, the emission spectra demonstrated a formation of peaks at -236 nm, 246 nm, 258 nm, and 283 nm, attributed to the exited states of \(\mathrm{NO_{\gamma}}\) species [4,11,32,39]. Figure 2:5 Emission spectra of the air plasma when \(\mathrm{V_{G}}\) was 90 V and 250 V.  
The existence of the NO \(\beta\)-system (B\({}^{2}\Pi-X^{2}\Pi\)) and NO \(\gamma\)-system (A\({}^{2}\Sigma^{+}-X^{2}\Pi\)) were observed in the wavelength range 200-300 nm for nitrogen or nitrogen-oxygen plasmas [32; 39]. The emission lines obtained at the 777.4 nm and 844.6 nm wavelengths are related to the existence of atomic oxygen (O), with transition lines of 3p\({}^{5}\)P \(\rightarrow\) 3s\({}^{5}\)S and 3p\({}^{3}\)P \(\rightarrow\) 3s\({}^{3}\)S, respectively (Figure 8b) [4; 19; 33; 42]. It should be noted that the intensity of the emission line attributed to atomic oxygen at 777 nm was more than two times lower than the intensity of the molecular nitrogen ion (Figure 9). The low-intensity peaks in the emission spectra of air plasma at -221 nm, -399 nm, and -434 nm indicate the formation of atomic nitrogen ions (Figures 8 and 9) [6]. It should be noted that several low-intensity emission peaks at 464 nm and 636 nm have been attributed to oxygen ions (O\({}^{+}\)) and molecular oxygen ions (O\({}_{2}\)+), respectively [6; 42]. The variation of the intensities of the main emission lines of the air plasma were determined and are presented in Figure 2:6 The variation of the intensity of the peaks related to the exited N\({}_{2}\) species observed at 337 nm and 353 nm versus the applied plasma generator voltage is presented in Figure 9, on the left. The results indicated that the emission line intensity related to the N\({}_{2}\) exited species at 336 nm increased from 18,200 counts to 62,600 counts with the increase in the V\({}_{\rm G}\) from 50 V to 130 V, respectively. The further increase in the voltage up to 250 V resulted in a slight decrease in the intensity values of the N\({}_{2}\) peak at 336 nm. It was found that the intensity of the N\({}_{2}\) emission line was reduced by -8.5%. Similar dependence trends of intensities versus voltage were obtained for all vibrationally excited N\({}_{2}\) molecules. The highest intensity of the atomic oxygen peak was obtained at the -777 nm wavelength. The increase in the voltage from 50 to 210 V increased its peak intensity from 315 counts to 5760 counts. However, the intensity of the emission peak was slightly reduced (by 2.5%) when the highest voltage was used (Figure 9). The intensity of the emission line at 391 nm, attributed to the molecular nitrogen ion (N\({}_{2}\)+), increased with the increase in the voltage.