# 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  
APPJs have nowadays become very important in many applications, especially for localized treatment of polymers and for various biomedical applications. Therefore, the knowledge on the generation of excited species and the control of their intensities during the treatment of materials with the APPJ is important. OES is a method that enables the real-time monitoring of the reactive plasma species during the operation of the APPJ. Here we report an example of the OES measurement along the plasma jet of one of the most commonly used and simple configurations of the APPJ (Figure 1). It is a single electrode APPJ having only one electrode. Alternatively, an additional electrode can be placed outside the dielectric tube, but in many practical cases, this is not necessary. A glowing plasma is stretching from the tip of the metallic electrode (a copper wire) mounted coaxially into the dielectric tube (made of Pyrex glass). The luminosity of gaseous plasma is the largest at the electrode tip and decreases with increasing distance. More details of this APPJ can be found in [3]. The discharge tube was made from Pyrex. The length of the discharge tube was 15 cm, and the inner diameters was 3 mm. A copper wire electrode was mounted inside the tube. A diameter of the copper electrode was 0.3 mm and the length of the electrode was the same as the length of the dielectric tube, i.e., 15 cm. The length of luminous plasma depends on the type of gas flow through the dielectric tube, the gas flow, and the properties of the power supply. In our case, a visible part of the plasma jet extended up to 3 cm from the exit of the discharge tube. The plasma jet shown in Figure 1 was fed with Ar gas with a purity of 5.0. The flow rate was adjusted to 1 slm. The powered copper electrode that was mounted inside the Pyrex dielectric tube was connected to an AC \begin{table}
\begin{tabular}{p{12.8 pt} p{128.0 pt} p{128.0 pt} p{128.0 pt}} \hline \hline
**REF** & **Gas Composition** & **APPJ Configuration** & **Findings by OES Diagnostics** \\ \hline \multirow{9}{*}{[39]} & & & The spatial distribution of radicals along the plasma plume was investigated versus the applied voltage and gas flow rate. & \\  & & & DBD type APPJ, square wave AC supply, peak-to-peak voltage 6 kV, flow rate: & Various He emission lines, O, OH, N\({}_{2}\) and N\({}_{2}\)\({}^{+}\) were observed. A strong decrease in emission intensity of He, O, and OH with a distance from the APPJ exit was observed. Opposite, N\({}_{2}\) and N\({}_{2}\)\({}^{+}\) reached a maximum at a distance of 5 mm from the exit and then gradually decreased. & \\  & & Powered hollow electrode, 2-10 kV\({}_{\text{PP}}\), 18 kHz, up to 0.33\% O\({}_{2}\) addition, flow 3 slm & The radiation intensity of RN and ROS radicals was strongly dependent on the O\({}_{2}\) concentration. For ROS radicals, different behavior of the emission intensity versus the O\({}_{2}\) concentration was observed than for RN radicals. & \\ \multirow{2}{*}{[41]} & & & & Production of NO radicals versus air admixture was investigated. The NO emission had a maximum at 0.1–0.2\% of air admixture in Ar. & \\  & & & & The influence of the MW power and gas flow rate on the emission intensity of plasma radicals was investigated. & \\ \multirow{2}{*}{[42]} & & & & A strong dependence of emission intensities on the power was observed. & \\  & & & OES measurements in UV and VUV for the various admixture of O\({}_{2}\) from 0 to 0.6\%. The following species were observed in the range 100–350 nm: O, O\({}_{2}\), NO, and OH. & \\  & & & & C, CN, CH, and C\({}_{2}\) species were observed in ethanol plasma. An increase in ethanol content in Ar caused a decrease in Ar lines and an increase in the relative amount of C\({}_{2}\). No significant dependence of the intensity ratio of excited species with the increase in RF power. & \\ \hline \hline \end{tabular}
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Table 1: Cont.