ON SPATIAL-DISTRIBUTION OF OPTICAL-EMISSION IN RADIO-FREQUENCY DISCHARGES

In this paper we present calculations of the spatial distributions of emission in model argon discharges which correspond to the conditions of argon discharges in the GEC rf reference cell but are relevant for other similar rf discharges. The calculations on the basis of the particle in cell (PIG) code show that transitions with high threshold energy are predominantly excited by secondary; electrons originating from the instantaneous cathode while the transitions with lower threshold energy are excited predominantly by electrons accelerated by sheath motion. In addition it is shown that the spatial distribution of 811 nm radiation of argon corresponds best to excitation by very low-energy electrons from the metastable state. The different kinetics of excitation of the three groups of transitions mentioned above, in conjunction with different energy dependences of the cross sections and special conditions when there is a significant if not dominant contribution of the gamma process in sustaining the discharge, give rise to the different spatial distributions of emission which are opposed to the intuitively expected distributions and raise questions about the applications of spatial distributions of emission in determining the sheath width and in diagnostic techniques such as actinometry.