The presence of molecules in the cold scrape-off layer of fusion experiments and industrial plasmas requires an understanding of the molecular dynamics in these low temperature plasmas. Suitable diagnostic methods can provide an insight in molecular processes in the plasma volume as well as for plasma surface interactions. A very simple but powerful technique is the molecular emission spectroscopy. Spectra are obtained easily, whereas interpretation might be very complex and relies on the availability of atomic and molecular data. However, the variety of results compensates for the complex analysis.
This paper presents examples for hydrogen which is one of the most simple molecules and occurs in the divertor plasma of fusion experiments as well as in laboratory plasmas. The molecular spectra provide information about rotational, vibrational and electronic populations in excited states. A correlation with the corresponding populations in the ground state and hence with plasma parameters, is obtained with collisional radiative modelling. Collisional radiative models for molecules require a larger amount of input data than models for atoms and ions, due to their manifold of excited states and extended coupling mechanisms with other species. The combination of spectroscopy with modelling allows for the determination of neutral particle temperature, vibrational population in the ground state, Te, ne, particle densities and fluxes of molecules and atoms. A further focus is led on the interpretation of Balmer lines which are influenced by molecular processes. Therefore, lines of the Balmer series can be used as a diagnostic tool for different species in ionising and recombining plasmas. The diagnostic methods and models are developed and checked in laboratory plasmas (low pressure discharges). From a comparison of hydrogen with deuterium plasmas isotope effects are observed. Applications to plasmas in plasma technology (negative ion sources) and to divertor plasmas of fusion experiments will be given. Another example are the hydrocarbon molecules. Here, collisional radiative modelling is replaced by dissociation modelling. Besides hydrocarbon plasmas, hydrocarbon molecules are formed also by the chemical erosion process of carbon materials in plasmas containing hydrogen. It will be shown, that the emission of radicals can be used to monitor the formation of methane and higher hydrocarbons. A method to quantify the amount of eroded material from the spectra is introduced as well.