# Optical Emission Spectroscopy as a Diagnostic Tool for Characterization of Atmospheric Plasma Jets
## 1 Introduction
Opposite to other authors, no N\({}_{2}\) emission was detected by OES because of special construction that prevented the mixing of the surrounding air. Electron density (8 \(\times\) 10\({}^{20}\) m\({}^{-3}\)) and gas temperature (<400 K) were also measured from the fine structure of the rotational bands of OH. OES was measured at various axial positions along with the plasma jet up to 12 mm (sampled every 2 mm). A maximum in the intensity of Ar lines was at 4 mm from the exit. The intensity of N\({}_{2}\) lines was increasing along with the jet because of the mixing of the surrounding gas. Significant OH emission was also detected. They found that APPJ is a source of UV-B (309 nm) and UV-A radiation (350, 380 nm), whereas no UV-C was detected in the range 250-280 nm. Irradiance in the UV range was determined to be 5 mW/cm\({}^{2}\) at the maximum power and the lowest axial distance. It can be concluded that several seconds of exposure of skin does not exceed the allowed exposure limit of 3 mJ/cm\({}^{2}\). An increase in OH was observed when water droplets were added to He APPJ. For Ar plasma, OH and N\({}_{2}\) species were dominant. For He plasma, N\({}_{2}\)\({}^{+}\) and O were observed. Emission profiles were measured versus a distance from the tube exit. Production of OH and O radicals was investigated. The production of OH and O was increasing with increasing Ar content in the gas mixture and with the applied voltage, but decreasing with increasing air flow. NO, OH, N\({}_{2}\) molecular, atomic N, He and O, and O\({}_{2}\)\({}^{+}\) species observed. Gas temperature was found to be \(\sim\)310 K, and the electron excitation temperature was \(\sim\)5420 K. OES was also used to investigate the generation of various species when the plasma jet was impinging onto the liquid. Besides He lines, also OH, O, N\({}_{2}\), and N\({}_{2}\)\({}^{+}\) species were detected. Rotational and vibrational temperatures from OH radical were determined in the active and afterglow region. Gas temperature slightly increased when O\({}_{2}\) was added or when moved from active to afterglow region. In the active plasma region, it was 310 and 340 K for He or He/O\({}_{2}\), respectively. Whereas \(T_{\rm vib}\) was 2200 or 2500 K, respectively. \begin{table}
\begin{tabular}{l l l} \hline
**REF** & **Gas Composition** & **APPJ Configuration** & **Findings by OES Diagnostics** \\ \hline \multicolumn{4}{l}{The collimating lens was positioned 140 mm from the end of the jet. The special construction of APPJ allowed the insertion of Ar(He)/CH\({}_{4}\) gas by a capillary tube into the main Ar/He flow, which prevents the mixing of the surrounding gas. In pure Ar plasma, the Ar and OH emission dominated. Additionally, in the Ar/CH\({}_{4}\) mixture, CH and C\({}_{2}\) bands were observed. In Ar/CH\({}_{4}\), CH was more dominant than C\({}_{2}\); whereas, in Ar/C\({}_{2}\)H\({}_{2}\), C\({}_{2}\) band was more intense. Opposite to other authors, no N\({}_{2}\) emission was detected by OES because of special construction that prevented the mixing of the surrounding air. Electron density (8 \(\times\) 10\({}^{20}\) m\({}^{-3}\)) and gas temperature (<400 K) were also measured from the fine structure of the rotational bands of OH. OES was measured at various axial positions along with the plasma jet up to 12 mm (sampled every 2 mm). A maximum in the intensity of Ar lines was at 4 mm from the exit. The intensity of N\({}_{2}\) lines was increasing along with the jet because of the mixing of the surrounding gas. Significant OH emission was also detected. They found that APPJ is a source of UV-B (309 nm) and UV-A radiation (350, 380 nm), whereas no UV-C was detected in the range 250-280 nm. Irradiance in the UV range was determined to be 5 mW/cm\({}^{2}\) at the maximum power and the lowest axial distance. It can be concluded that several seconds of exposure of skin does not exceed the allowed exposure limit of 3 mJ/cm\({}^{2}\). An increase in OH was observed when water droplets were added to He APPJ.