# Optical Emission Spectroscopy as a Diagnostic Tool for Characterization of Atmospheric Plasma Jets
## 3 Examples of Using OES for Gas Temperature and Electron Density Measurements
They managed to estimate the electron density inside such a plasma streamer to approximately \(10^{19}\) m\({}^{3}\). The rotational and vibrational temperatures, as calculated from the high-resolution OES spectrum of the OH radical, were found dependent on the population of the excited states. A strong overpopulation of highly excited rotational states was observed, so the rotational temperature is not defined for such discharges. Such a rather unexpected population of excited states was explained by the quenching of excited OH radicals which, according to [PERSON] et al., depended on the excitation level. High electron densities were also reported for the APPJ similar to the one in Figure 2 sustained in Ar with an admixture of organic gas, in particular, tetraethyl orthosilicate (TEOS) [13]. The electron density peaked at approximately \(10^{23}\) m\({}^{-3}\) in a short time during the evolution of streamers. This value is much larger than what was reported by other authors. For example, [PERSON] et al. [31] reported four orders of magnitude lower electron density. As already explained, the discharges sustained by rather low-frequency AC power supplies cause the formation of numerous streamers of short duration where the peak electron density is difficult to measure. One should beware of averaging results over the time scale much longer than a streamer duration. The intensity of spectral features followed the evolution of the electron density. The OH band at 309 nm prevailed despite the fact that no water vapor was added to the gas mixture intentionally. The peak power absorbed by plasma enabled extensive radicalization of the organic precursors, and even C\({}_{2}\) dimers were detected in the optical spectrum. [PERSON] et al. [13] also determined rotational temperature calculated from CH, N\({}_{2}\), or OH radicals. This was the first time that CH radical was used for the estimation of \(T_{\rm g}\). Despite the rather large average power of 11.5 W, the rotational temperature was between approximately 410 K, if calculated from OH, and 550 and 590 K if calculated from N\({}_{2}\) and CH radicals, respectively. The authors showed that taking CH radical leads to an overestimation of \(T_{\rm g}\). Taking into account the results reported by previously cited authors, it is clear that the deduction of the gas temperatures involved in APPJs may not be straightforward. The APPJ device used by [PERSON] et al. [13] was also applied for the deposition of thin-films from Ar/TEOS, where knowing \(T_{\rm g}\) is one of the key parameters needed to have control over the deposition process. An RF discharge operated at 13.56 MHz at rather high power was also used for sustaining a micro-plasma jet by [PERSON] et al. [32]. As mentioned earlier, such plasmas operate in continuous mode. The authors found the electron density close to 10\({}^{21}\) m\({}^{-3}\), although the neutral gas temperature was below 400 K. They used a coaxial configuration, and the plasma plume expanded a few mm from the tip of the powered electrode. They managed to assure negligible diffusion of air into the plasma plume, so the optical spectra consisted only of Ar lines and OH band. A small addition of N\({}_{2}\) into the gas caused significant radiation of the N\({}_{2}\) second positive band and suppression of OH radiation, similar to what is observed in Figure 3.