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 part icle in cell (PIG) code show that transitions with high threshold ener gy are predominantly excited by secondary; electrons originating from the instantaneous cathode while the transitions with lower threshold e nergy are excited predominantly by electrons accelerated by sheath mot ion. In addition it is shown that the spatial distribution of 811 nm r adiation of argon corresponds best to excitation by very low-energy el ectrons from the metastable state. The different kinetics of excitatio n of the three groups of transitions mentioned above, in conjunction w ith different energy dependences of the cross sections and special con ditions when there is a significant if not dominant contribution of th e gamma process in sustaining the discharge, give rise to the differen t spatial distributions of emission which are opposed to the intuitive ly expected distributions and raise questions about the applications o f spatial distributions of emission in determining the sheath width an d in diagnostic techniques such as actinometry.