THE INFLUENCE OF METASTABLES ON THE PLASMA-INDUCED EMISSION IN A HELIUM RF DISCHARGE

Space- and time-resolved emission spectroscopy is commonly used to inv estigate heating mechanisms in rf discharges (plane parallel plate con figuration). The metastable density in these discharges is several ord ers of magnitude lower than the density of the ground-state atoms; how ever, the electron collision excitation cross sections of some helium levels out of the metastable levels exhibit values which are several o rders of magnitude larger and have much lower thresholds than those fo r the ground state. It is therefore questionable whether stepwise exci tation from metastable levels can be neglected in the interpretation o f the spatiotemporal plasma-induced emission of rf discharges. To our knowledge only the excitation of ground state atoms has been taken int o account in model calculations to this date. With respect to the ener gy balance of such a discharge, this assumption is probably correct. H owever, with respect to the emission of light, our computational resul ts show that some specific levels are excited significantly by electro n collisions with metastables. We will show in this contribution, on t he basis of our measurements, that this effect is responsible for the observed emission profiles of some lines. Consequently, the interpreta tion of the emission profiles in helium rf discharges has to take acco unt of the role of metastables.