Ruhr University Bochum Institute for Electrical Engineering and Plasma Technology Prof. Dr.-Ing. Peter Awakowicz Universitätsstraße 150 Tel: 0234/32-23062 44801 Bochum Web: www.aept.rub.de Optical Emission Spectroscopy (OES) - What You See Is What You Get Marcel Fiebrandt [email protected] 1 Motivation All plasmas emit radiation depending on their gas com- position and plasma parameters. The goal of optical emission spectroscopy (OES) is measuring and analy- sing this radiation to get an insight into the plasma. Fig. 1: Colors of plamas in a DICP due to different ga- ses (from left to right: H 2 ;N 2 ;O 2 ) The significant advantage of OES is its invasive mea- surement process. By just measuring the radiation from the plasma there is no need to bring a probe system in contact with the plasma (like Langmuir-Probe or Multi- pol Resonance Probe) which might influence the plas- ma itself. Furthermore, OES enables one to investigate very small plasmas, often smaller as probes themsel- ves. Therefore, it is often the only usable measurement technique in addition to laser absorption spectroscopy. Depending on the plasma, radiation from the vacuum UV (< 200 nm), UV (200 nm - 380 nm), visible (380 nm - 780 nm) or infrared region (> 780 nm) can be detected and analyzed. Fig. 2: VUV and UV spectrum of an Ar/N 2 /O 2 DICP- Discharge [1] 2 Measurement Modern spectrometers with CCD-Chips allow fast mea- surements of spectra over a broad wavelength region. Combined with an optical fibre, the emitted plasma ra- diation is guided to the entrance slit of the spectrome- ter, making it very easy to measure spectra from diffe- rent positions and angles. Furthermore, if the spectro- meter is absolutely calibrated [2], it is possible to mea- sure the number of photons per wavelength, second and volume emitted by the plasma. As the excitation of atoms and molecules in the plasma depends on the parameters describing a plasma, like electron tempera-