# Optical Emission Spectroscopy as a Diagnostic Tool for Characterization of Atmospheric Plasma Jets
## 5 Summary  
An atmospheric pressure plasma jet is useful for localized surface treatment of various materials, as well as tissues. Therefore, knowing the interactions of both radiation and chemically reactive plasma species with the surface is important. Characterization of the APPJ is recommended before any application to control surface reactions and reproducibility of the results. The simplest technique is optical emission spectroscopy (OES). This technique enables space-resolved determination of the radiative species in the APPJ. Usually, OES is mostly used to identify species and monitor reaction products when the plasma interacts with the surface being treated. However, the application of OES to measure the gas temperature is rising, although it requires more in-depth knowledge and the use of computer models. Spectrometers are available with different spectral resolution. Spatial and temporal resolution of the spectrometers are continuously increasing with technological development. Recently, [PERSON] et al. [70] performed spatially resolved OES using integrating (Ulbrecht) sphere for the first time. This ensured many diffusive reflections of the light in the spherical cavity thus enabling uniform and spatial resolved OES. When comparing to the regular use of OES, similar emission and distribution of excited species was detected, however, species in the visible and near infrared (VIS/NIR) region and especially those in UV region were detected with 4-5 times higher intensity. Nevertheless, because plasma is a rich source of VUV photons, the major drawback of OES is still inability to measure in the VUV range, where more expensive VUV spectrometers are needed. Detailed information about the behavior of different reactive species can be obtained by comparison of the OES spectra with results of more thorough plasma characterization with other techniques reported in the literature. Several authors have reported detailed studies using more expensive and demanding characterization techniques but similar discharges. Such a comparison is useful for the interpretation, but one should beware of huge discrepancies summarized in this paper. The APPJ is often sustained in Ar without the intentional addition of a reactive gas. It was shown that the traces of water (that have been adsorbed in the discharge or connecting tubes) might play a dominant role in surface reactions. Any APPJ plasma operating in the atmosphere thus contains OH radicals. The emission from OH radicals is much stronger in pure Ar plasmas than in pure He plasma. The OH radicals are among chemicals of the highest oxidation potential; therefore, they interact intensively with a material surface causing rapid activation. Furthermore, the OH radicals have a rather large internal energy (the vibrational temperature often exceeds 1000 K) what contributes to the chemical reactivity of these radicals. Various OH densities and its rotational and vibrational temperatures were reported by different authors, but as a general rule, the OH density decreases quickly with increasing distance from the APPJ powered electrode. Some authors reported the OH densities well over 10\({}^{20}\) m\({}^{-3}\) what should be sufficient for saturation of a polymer surface with functional groups. The optical emission spectroscopy will not give the absolute density of the OH radicals in the ground state but will rather represent a useful technique for detection and estimation of the density, as explained in appendix A. For the determination of the absolute values, the optical absorption spectroscopy is the method of choice. The absorption spectroscopy comes in various configurations. Because the absorption cross-section for 4 eV photons from a laser on OH radicals is rather small, multiple passes are needed to obtain reliable results. An alternative is the simultaneous absorption of two or three photons of lower energy and observation of the fluorescence. The technique is known as two-atom LIF (TALIF).