# 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  
The temperatures can be deduced from the optical spectra only when high-resolution spectrometers are used. Furthermore, more knowledge is needed and application of the simulation software. That is why the rotational and vibrational temperatures are rarely reported. Therefore, the spectra shown in Figure 3 will not reveal the temperatures because of the insufficient resolution of the spectrometer. A user of APPJ should, however, beware when concluding about the temperatures involved within the plasma jet. The temperature may differ depending on which emission line (OH(A) or N\({}_{2}\)(C)) is used for its calculation [13]. Furthermore, the temperatures reported in the literature may significantly differ, although similar jets and plasma parameters were used. [PERSON] et al. measured rather high-resolution OES spectra to deduce the rotational (\(T_{\text{rot}}\)) and vibrational (\(T_{\text{vib}}\)) temperatures of OH radicals [29]. For \"pure\" Ar, the temperatures were found independent from the distance from the electrode at the values of \(T_{\text{rot}}\) = 450 K, and \(T_{\text{vib}}\) = 1500 K. An addition of 500 ppm of water vapor did not have a significant effect on the temperatures away from the electrode, but near the electrode, the vibrational temperature was approximately 2000 K. When 7600 ppm of H\({}_{2}\)O was added, \(T_{\text{rot}}\) increased to approximately 800 K rather far from the electrode, where \(T_{\text{vib}}\) was almost 5000 K. The values were even larger next to the electrode. High \(T_{\text{rot}}\) and \(T_{\text{vib}}\) temperatures were also reported by [PERSON] et al. for the case of the APPJ sustained with a microwave (MW) discharge (i.e., in the continuous mode) [27]. In almost pure Ar, \(T_{\text{rot}}\), and \(T_{\text{vib}}\) temperatures were practically equal at approximately 900 K. The addition of 1 vol.% H\({}_{2}\)O caused an increase in both temperatures to approximately 1100 K. The huge discrepancy between the results of [PERSON] et al. [27] and [PERSON] et al. [29] may be explained by different discharges. While [PERSON] et al. used a continuous plasma, [PERSON] et al. selected pulsed discharge with electron streamers, as shown schematically in Figure 2. Obviously, the pulsed plasmas created by rather low-frequency discharges cause overpopulation of highly vibrational excited states, whereas continuous plasmas cause almost equal \(T_{\text{rot}}\) and \(T_{\text{vib}}\). In any case, the temperatures are large, in many cases well above the softening or melting point of polymers. Such hot OH radicals interact abruptly with polymer surfaces, causing functionalization and chemical etching at prolonged treatment times. These effects should be taken into account at any attempt to activate a polymer surface by atmospheric pressure plasmas. Opposite to [PERSON] [27] and [PERSON] [29], [PERSON] et al. [37] reported a very low \(T_{\text{rot}}\) of only approximately 330 K. Such low \(T_{\text{rot}}\) was also determined by [PERSON] et al. [38] for DBD-type APPJ in He plasma with small O\({}_{2}\) admixture. Plasma was characterized in the active plasma region and in the afterglow region. It was found that gas and vibrational temperatures increased when O\({}_{2}\) was added. The temperature was also slightly higher in the afterglow region compared to the active plasma region. Gas temperature was found to be 310 and 340 K for He and He/O\({}_{2}\), respectively, whereas \(T_{\text{vib}}\) was 2200 and 2500 K, respectively. [PERSON] et al. [37] determined rotational temperature using N\({}_{2}\) line at 337 nm, whereas [PERSON] et al. [38] used OH. As will be shown later, the calculated temperature may differ depending on the radical used for calculation. [PERSON] et al. [31] also performed a detailed characterization of pulsed discharges using the same equipment as [PERSON] et al. [29]. They used pulsed discharges of a duration of approximately a microsecond.