## 1 Introduction  
Thin film deposition has great significance in industrial and medical processes, and numerous studies are concerned with the development of thin-film processes for specific applications [1, 2]. Plasma spraying by means of an atmospheric pressure plasma jet (APPI) allows the deposition of thin layers of metal, metal oxide or polymers and on temperature-sensitive substrates, such as paper or wood. In the process, particles are melted in the plasma of an atmospheric pressure gliding plasma arc jet and applied to the surface [3, 4, 5]. The plasma source and parameters ensure low particle temperatures at the nozzle exit compared to a thermal plasma spray, and at working distances of \(\geq\)20 mm no thermal impact to the specimen occurred. The process is also termed \"cold plasma spraying\" [6, 7]. For broad application, understanding the underlying mechanism and influence of the relevant parameters, process stability and monitoring are necessary. Appropriate methods need to be non-invasive, reliable and fast to detect changes in plasma properties in real time and under process conditions. Optical emission spectroscopy (OES) is well established for the characterization of APPJ because it provides insight into plasma energy distributions and kinetics and is interference-free. OES experiments showed that gliding arcs exhibit an initial quasi-equilibrium zone close to the electrodes, followed by a non-equilibrium zone with electron temperatures about 10 times higher than rotational and translational temperatures [8]. In general, studies showed similar trends for different types of APPJs, including an increase in plasma temperature and electron density with input power or a decrease in plasma temperature with increasing axial distance. Changes in gas composition, well studied for the influence of water, lead to a change in kinetics and therefore a strong variation in plasma temperatures, electron densities, and excited species densities can be observed [9, 10, 11, 12]. The presence of substrates can have similar effects, but many jetsare monitored under laboratory conditions, and not many studies focus on substrate interactions [13]. Recent developments have focused on the application of OES for the monitoring and controlling of plasma. Line-ratio methods and simple kinetic models allow evaluation in real time for selected processes [14,15]. To establish OES as a tool to monitor cold plasma spray, fundamental investigations are first necessary. In this study, OES was used for the downstream characterization of the effluent plasma of an APPJ for the thin film deposition of metals, metal oxides or polymers. Beside the determination of reactive species and their development being dependant on the distance from the nozzle and different gas flows, special attention was paid to the determination of plasma temperatures. A rotational temperature, determined by an analysis of the molecular spectrum of a species such as N\({}_{2}\), N\({}_{2}^{+}\), NO or CN, is commonly assumed to be equal to the gas temperature in atmospheric pressure plasmas as long as certain conditions are fulfilled [16,17]. Special care has to be taken in the selection of spectral bands to avoid spectral overlapping of different species. In addition, knowledge of rotational temperature, electron density and electron temperature, also referred to as electron energy, are fundamental. While determining electron density by the Stark effect is possible using OES if the gas temperature is known and the resolution of the spectrometer is sufficient, determining electron energy is sophisticated and requires a valid kinetic model [18,19]. On the other hand, excitation temperatures can easily be derived by line-ratio methods. The general trend for both, electron and excitation temperatures, is the same, while for non-LTE plasmas the electron temperature is higher than the excitation temperature [20,21].