The influence of metastables on the plasma-induced emission in a helium rf discharge

Abstract

Space- and time-resolved emission spectroscopy is commonly used to investigate heating mechanisms in rf discharges (plane parallel plate configuration). The metastable density in these discharges is several orders of magnitude lower than the density of the ground-state atoms; however, the electron collision excitation cross sections of some helium levels out of the metastable levels exhibit values which are several orders of magnitude larger and have much lower thresholds than those for the ground state. It is therefore questionable whether stepwise excitation from metastable levels can be neglected in the interpretation of the spatiotemporal plasma-induced emission of rf discharges. To our knowledge only the excitation of ground-state atoms has been taken into account in model calculations to this date. With respect to the energy balance of such a discharge, this assumption is probably correct. However, with respect to the emission of light, our computational results show that some specific levels are excited significantly by electron collisions with metastables. We will show in this contribution, on the basis of our measurements, that this effect is responsible for the observed emission profiles of some lines. Consequently, the interpretation of the emission profiles in helium rf discharges has to take account of the role of metastables.