# 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
The absorption cross-section for H\({}_{2}\)O molecules was also provided by [PERSON] et al. and may be as high as 10\({}^{-17}\) cm\({}^{2}\). For He plasma, the intensive radiation within 60 and 100 nm was suppressed even by a very small addition of a reactive gas. For example, the addition of 0.01% O\({}_{2}\) caused suppression of the He radiation by a factor of two. Unfortunately, no data for water vapor were provided by [PERSON] et al. Based on the discussion above, it is possible to conclude that a simple plasma jet operating in the configuration of Figure 1, may be a source of radiation in the far UV range, but this cannot be probed with the simple spectrometer. The experimental configuration with a VUV spectrometer, as adopted by [PERSON] et al., is highly recommended, but difficult to realize. According to this literature review and our results, we can conclude that OH radicals are the most important reactive species in the APPJ. As OES is a qualitative technique, it enables investigation of the relative variation of the density of plasma species along the axis. However, for quantitative determination of the densities, other techniques must be applied. The OH density profile along the plasma jet is usually measured by laser-induced fluorescence (LIF) [53; 54; 55; 56] or two-photon absorption LIF (TALIF) [57; 58; 59]. Other methods, such as cavity ringdown spectroscopy (CRDS) [27; 60; 61; 62] and absorption spectroscopy [57; 63] can be used as well. [PERSON] et al. measured the concentration of the OH radicals in the humidified He discharge sustained in an RF driven APPJ by using vacuum ultra-violet high-resolution Fourier-transform absorption spectroscopy and ultra-violet broad-band absorption spectroscopy [57]. They found an extensive dissociation of H\({}_{2}\)O molecules upon plasma conditions. The density of OH radicals was approximately 3 \(\times\) 10\({}^{20}\) m\({}^{-3}\). Such a large density expanded several cm along the discharge channel. An order of magnitude lower OH density was observed by [PERSON] et al. [62] in a pulsed He discharge with CRDS method. Whereas [PERSON] et al. [56] obtained even lower densities of approximately 1 \(\times\) 10\({}^{18}\) m\({}^{-3}\) with LIF. In another paper, [PERSON] et al. [55] measured the OH density in the humidified He pulsed discharge. The H\({}_{2}\)O content in He gas was varied between 0 and 800 ppm. A maximum in the OH density peaking at 6 \(\times\) 10\({}^{18}\) m\({}^{-3}\) was found at the H\({}_{2}\)O content of 200 ppm. Additionally, [PERSON] et al. measured the variation of the OH density along the axis of the APPJ [27; 60] by using CRDS method. [PERSON] et al. [60] measured the OH density in Ar, Ar/N\({}_{2}\), and Ar/O\({}_{2}\) APPJ discharge at various powers and flow rates. The OH density was decreasing along the jet. The OH density also strongly depended on the power and it was increasing with the increasing power. For Ar plasma, the OH density ranged from 1.3 \(\times\) 10\({}^{18}\) to 1.1 \(\times\) 10\({}^{22}\) m\({}^{-3}\), depending on particular conditions. For Ar/N\({}_{2}\) it was in the range between 4.1 \(\times\) 10\({}^{19}\) to 3.9 \(\times\) 10\({}^{21}\) m\({}^{-3}\), whereas for Ar/O\({}_{2}\) it was between 7.0 \(\times\) 10\({}^{18}\) to 4.6 \(\times\) 10\({}^{22}\) m\({}^{-3}\)[60].