# Optical Emission Spectroscopy as a Diagnostic Tool for Characterization of Atmospheric Plasma Jets
## 1 Introduction
The spectral response is far from being constant, so calibration with a standard source is recommended at any attempt to use OES for plasma characterization. The standard spectrometers will therefore detect radiation arising in a range of photon energies between the upper and lower energy levels (1 and 6 eV). Most atomic transitions such as Ar, He, O, N, H, to the ground state are below 200 nm. These transitions are, therefore, invisible by standard spectrometers. The transitions to the ground state are normally much more extensive than to other states, so one misses most radiation when using standard spectrometers. This limitation should be taken into account at any attempt to interpret the plasma spectra acquired by OES. The VUV spectrometers (wavelength 100-200 nm) are increasingly popular in plasma science, but one should keep in mind that the radiation of photon energy above the dissociation energy of oxygen molecules (5.2 eV) is absorbed in air, what complicates the experimental setup. Still, knowing the above limitations, one can find OES a useful, simple, and inexpensive technique, so it is widely used for the basic characterization of atmospheric plasmas. OES can also enable the determination of the gas temperature, electron density, and electron excitation temperature; however, this already requires more in-depth knowledge. The methods for deduction of these plasma properties are not trivial and are beyond the scope of this paper. Gas temperature is assumed to be similar to rotational temperature (\(T_{\mathrm{g}}\approx T_{\mathrm{rot}}\)). Gas temperature \(T_{\mathrm{g}}\) is thus determined from the calculation of rotational temperature \(T_{\mathrm{rot}}\) of diatomic molecules considering Boltzmann distribution of rotational states [13; 25; 26]. For the APPJ plasmas, OH(A) and N\({}_{2}\)(C) excited states are usually used for the determination of the rotational states. However, the obtained \(T_{\mathrm{rot}}\) may differ if calculated from OH or N\({}_{2}\), and it often leads to an overestimation of \(T_{\mathrm{g}}\)[13]. The spatial resolution depends on the lenses mounted at the end of the optical fiber. Normally, the radiation is collected from a solid angle of few degrees. The acquisition time depends on the radiation intensity. Widely used inexpensive spectrometers will have an adjustable acquisition period between approximately 1 ms and several s. Obviously, simple spectrometers will be particularly useful for characterization of plasma in a continuous mode. When the plasma is in the form of streamers of shelf-time well below a ms, the OES will give values averaged over numerous streamers. The present paper explains some basic properties of the OES technique and gives a review of published literature, which is summarized in Table 1 and further discussed in the main part (Sections 2 and 3). Moreover, additional details regarding the APPJconfiguration, flow rate, voltage, power, etc., are given in Table 1, making the comparison of various results easier. In the main part, some authors' own experiments are presented and discussed in a view of the cited literature, which gives a more comprehensive insight in the phenomena likely to occur in APPJs.