Non-thermal Effects in Stark Broadening*

When an atom is immersed in a plasma the energy levels are broadened and shifted by the interaction with the fluctuating electric field of the plasma. We discuss the effect of this field on the spectral line shape by the use of results from plasma kinetic theory. In particular we show that in the equilibrium plasma the dominant effect is from single-particle collisions although correlation effects lead to a different result from the usual impact theory. In nonequilibrium plasmas the collective modes can become important and lead to appreciable satellites on the forbidden lines. In the case of a two-temperature electron gas the plasma wave effect is considerably enhanced. The possibility of observing the satellites in unstable plasmas is discussed and the relationship to quasilinear theory is given. In particular, we show that in isotropic systems the line shape can be used to directly observe the turbulent spectrum. I N T R O D U C T I O N IN AN earlier paper'') we discussed the fact that the effect of electron correlations on Stark broadening had been incorrectly treated in the usual impact theory.'" We showed that a well-known result of plasma kinetic theory leads to an expression for (E(r) . E(0)) which could be interpreted as dynamic double-screening as opposed to the static (or dynamic) single-screened expression proposed by GRIEM et al.") We indicated very briefly the effect of the additional screening on Griem's function and mentioned the possible effects of collective modes. In the past year there has been increasing interest in the effects of nonequilibrium plasma^'^.^) on line shapes and other applications of plasma kinetic theory'5) to spectral line shapes. The purpose of this paper is to give a more complete explanation of the "doublescreened" result and to discuss the effects of nonequilibrium plasmas on the line shape. We begin by showing that if strong interactions of the atom with the plasma are ignored the line shape can be written in terms of the electric field autocorrelation function (E(t). E(0)) in the plasma. * This work was supported in part by the advanced Research Projects Agency of the Department of Defense t Of the National Bureau of Standards and the University of Colorado. and was monitored by Army Research Office-Durham under Contract No. DA-31-124-ARO-D-139.