# 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
gradual characterization with a reasonable vertical resolution by moving the optical fiber vertically along the jet, as shown in Figure 2.  
A typical OES spectrum of Ar plasma acquired close to the electrode tip of the APPJ device (i.e., 2 mm from the exit) is shown in Figure 3 (lower curve). Moreover, the OES spectra acquired 10 and 25 mm from the electrode tip are shown in Figure 3 as well. Figure 3 reveals transitions of neutral Ar atoms that correspond to the relaxation of highly excited states to the metastable state. The transitions are in the red part of the spectrum. There are also transitions to the ground state, but they appear in the far UV range, which cannot be probed with the simple optical spectrometer. Interesting enough, Ar lines are not the only spectral features in plasma 2 mm from the exit of the dielectric tube. The most intensive feature appears at the bandhead of approximately 309 nm, and it corresponds to the transition of the OH radicals from the electronically excited to the ground state. Furthermore, there is a band of lines in the near UV range that corresponds to the transition of highly excited neutral nitrogen molecules in C\({}^{3}\Pi_{\text{u}}\) state to N\({}_{2}\) molecules in B\({}^{3}\Pi_{\text{g}}\) state. This transition is called 2 nd positive system. Details on the most common transitions of various atoms and molecules likely to occur in the APPJ are very well presented in the paper of [PERSON] et al. [45]. Spectral features as observed in Figure 3 in the spectrum acquired close to the APPJ exit also persist in the spectra recorded at longer distances from the exit of the discharge tube, however, their intensity is significantly lower, and also their ratio strongly depends on the distance from the dielectric tube. The axial distribution of the major spectral features versus the distance is shown in Figure 4.  
[PERSON] et al. [30] used a device similar to what is shown in Figure 1 but powered it with a DC pulsed discharge. In such discharges, the streamers, as shown in Figure 2, shrink to bullets of a spatially confined dense plasma, which propagate away from the powered electrode at a velocity of the order of \(10^{4}\) m s\({}^{-1}\). The visible appearance of gaseous plasma is not much different from the one shown in Figure 1, but the electrical properties are completely different. Despite the difference, the optical spectra acquired by [PERSON] et al. are similar to those in Figure 3. In the optical spectra acquired by [PERSON] et al., the OH line at 309 nm was dominant, and the N\({}_{2}\) band in the near UV range was clearly visible, too (Figure 5). They also provided images of the discharges taken with the fast camera to show  
Figure 3: The OES spectra recorded at various distances from the exit of the discharge tube. The spectra were normalized to Ar line at 764 nm. the evolution of the plasma bullets. The optical spectra taken in such cases obviously give values averaged over numerous such localized plasma bullets. Additionally, also [PERSON] et al. [28] and [PERSON] et al. [35] observed similar spectra as shown in Figure 3 for the dielectric-barrier discharge (DBD) type APPJ feed with pure Ar as well as [PERSON] et al. [33]. [PERSON] et al. [28] used the APPJ very similar to the device shown in Figure 1. Intensive OH radiation was observed even in pure Ar, and suppressed by the intentional addition of other gases. They provided possible channels for the formation of hydroxyl radicals. The OH radical can be generated by a collision of the water molecule and an electron (electron-impact dissociation), or a metastable Ar atom, or by the reaction of an excited oxygen atom with a water molecule, which was found the dominant mechanism by [PERSON] et al. Their arguments were based on the papers of [PERSON] et al. [46] and [PERSON] et al. [47]. In addition, [PERSON] et al. [36] used OES to investigate the production of OH and O radicals in air/Ar/H\({}_{2}\)O plasma and found that Ar plays a significant role in their production.