# Optical Emission Spectroscopy as a Diagnostic Tool for Characterization of Atmospheric Plasma Jets
## 2 OES Characterization of the Reactive Species along the APPJ Plasma Jet
Based on this observation, [PERSON] et al. concluded that the yield of OH radicals was the highest at such a low water admixture. [PERSON] et al. elaborated the evolution of optical spectra along the APPJ operating in the semi-continuous mode [33]. They used an RF supply operating at a frequency of 1.1 MHz. They observed gradual decay of the OH and N\({}_{2}\) spectral features with the distance from the powered electrode. The OH radiation reached the peak value 2 mm away from the electrode, while nitrogen about 10 mm. The observations are, therefore, similar to those presented in Figures 3, 5 and 6. They also managed to measure the axial and radial temperature profiles using a glass fiber temperature probe. As expected, the highest gas temperature was observed where the luminosity was the highest, but the plasma tip temperature at the axis was about 48 \({}^{\circ}\)C. Rather strong radial gradients were observed, and the gas remained at room temperature a few mm from the visible plasma jet. These results indicate that the rotational temperatures, as reported by the above-cited authors, may not be representative for the estimation of the neutral gas kinetic temperature. Production of OH radicals in the bullet-like APPJ was recently elaborated also by [PERSON] and [PERSON] [49]. The APPJ was sustained in He gas of purity 99.9999% with a pulsed discharge powered with a 10 kV supply. The voltage rise-time of 60 ns only assured for almost perfectly-shaped bullets propagating from a tungsten electrode which was placed into a quartz glass tube, similar to Figure 2.4 The emission from OH radicals was much larger inside than outside the tube, confirming that the water vapor originated from the walls inside the discharge tube. The authors explained the extensive radiation from OH radicals by electron-impact dissociation and subsequent excitation rather than the influence of He metastables. The formation of the bullet-like discharge was further elaborated by [PERSON] et al. [50]. They performed a detailed characterization of a plasma jet useful for blood coagulation. Plasma was sustained in He or Ne using a low-frequency discharge, which provided current pulses of a duration of approximately 1 \(\upmu\)s. No additional gas was added intentionally, but the OES spectra revealed nitrogen bands even within the dielectric tube despite the rather large gas flow of 2 slm. At a distance of 5 mm from the exhaust, the spectral features corresponding to N\({}_{2}\) and O transitions prevailed over the radiation from He atoms. Unfortunately, the spectrometer used by [PERSON] et al. [50] did not allow for measuring the OH line at 309 nm to compare the intensities like in Figure 2.5 In another paper, the same group reported the thermal effects caused by the treatment of biological matter with such an APPJ [51]. They found experiments with biological materials unfeasible because of the unknown thermal properties, so they used metallic samples and found significant differences for grounded or floating objects. The heating rate depended on the distance from the electrode, indicating the loss of the discharge power along the APPJ. [PERSON] et al. [39] performed an investigation of the spatial distribution of excited and ionized species along the bullet-like APPJ sustained in He at different flow rates. The emission of H, O, and OH strongly decreased with the distance from the APPJ exit, whereas N\({}_{2}\)+ and N\({}_{2}\)* reached a maximum at few mm from the tube exit. [PERSON] et al. [40] investigated the production of reactive oxygen species (ROS) such as OH and O and reactive nitrogen species (RN) such as N\({}_{2}\) and N\({}_{2}\)+ in He plasma with various oxygen admixtures up to 0.33%. They found a strong dependence of ROS and RN species versus O\({}_{2}\) admixture. The different behavior of the emission intensity of ROS and RN radicals versus O\({}_{2}\) concentration was observed. In the case of RN radicals, N emission exhibited a maximum at 0.05%, whereas N\({}_{2}\) at 0.1% of O\({}_{2}\) addition.