## 3 Results and Discussion
### Optical Diagnosis of the Plasma Source
#### 3.4.2 OES Measurements  
Emission spectra in the 200-900 nm spectral range of the plasma jets show signatures of molecular and atomic excited species (Figure 10). The molecular bands are assigned to nitrogen oxide (NO\({}_{7}\) system), hydroxyl radicals (OH), neutral nitrogen molecules (N\({}_{2}\)) and nitrogen molecular ion (N\({}_{2}^{+}\)). The NO\({}_{7}\) system is observed in the 200-280 nm region with characteristic bands, as for the OH radical, their signature is between 306-310 nm. Bands of N\({}_{2}\) are observable in the spectral range of 315-380 nm and 399-405.9 nm. The N\({}_{2}^{+}\) have bands at 391.4 nm, 427.8 nm and 470.0 nm. The atomic lines form the emitted spectra are assigned to helium atoms (lines at 588.8 nm, 668.5 nm, 706.6 nm and 727.5 nm) or to argon atoms (lines between 680 to 850 nm) as working gas, but also to oxygen atoms (lines at 777.8 nm and 845.5 nm) as products of ambient O\({}_{2}\) and H\({}_{2}\)O dissociation [31, 65, 67, 80, 81, 82]. Figure 9: VUV spectra of the discharge in argon (**left**) and helium (**right**) for an applied voltage of 8 kV\({}_{pp}\) and a distance to the MgF\({}_{2}\) window of 4 mm. Ar-CAP generated a spectral radiance of up to 350 mW/(nm mm\({}^{2}\) sr) at around 125 nm. In the 220-280 nm spectral region (the UV-C), we observed only for the Ar-CAP the emission signatures of the nitric oxide molecular band (NO\({}_{7}\), in Figure 10), well known for their role as signaling molecules involved in wound healing [83]. The OH and O lines are observed for He-CAP and Ar-CAP, while the line intensity was higher in Ar-CAP. This observation supplements the data acquired from VUV measurements as well as the species density measurements. Another characteristic of the OES spectra of the He-CAP is that the presence of excited nitrogen species is dominantly observed for both N\({}_{2}\) and N\({}_{2}^{+}\) compared to Ar-CAP due to the discharge mechanisms, mainly based on penning effect for helium. In particular, the nitrogen molecular ion is observed in helium CAP as the most intense line. Nonetheless, in the Ar discharge, the NO bands are observed at a lower temperature than in He discharges. Variations of the atomic lines and bands emitted intensities for the plasma jet operating in air, depending on the working gas, can be easily observed in Figure 10. Moreover, the importance of such plasma active/reactive species is to be considered also from the energetic point of view of these species. Thus, energy values from 4 eV up to 23 eV are attributed to OH radicals, N\({}_{2}\) and N\({}_{2}^{+}\) molecules, hydrogen He, Ar and O atoms, as reported by [78,84]. The results until now show that both devices operated at comparable power values, identical flow and voltage settings generate species with a different efficiency. One way to explain this tendency is via the basic plasma properties of rotational temperature and by considering the given temperatures involved in the chemistry. The ratio between O\({}_{3}\) and NO\({}_{x}\) is known to depend on the temperature [85]. However, our measurement of the surface temperature, via a temperature optical fiber, in Figure 7 shows a temperature range from 35 \({}^{\circ}\)C up to 135 \({}^{\circ}\)C for the He-CAP and a more or less constant temperature for the Ar-CAP in between the helium range. According to neutral gas temperature alone, this effect of species densities can not be explained. Since the studied plasma source is at atmospheric pressure, a non-thermal plasma is assumed. Therefore, the temperature of the electrons is significantly higher than the temperature of the ions and of the neutrals. A frequently used assumption considers the rotational temperature of neutral molecules to be close to the neutral temperature. Therefore, a further investigation of the rotational and vibrational temperatures in He-CAP and Ar-CAP will be performed.