KINETIC-THEORY OF SPECTRAL-LINE FORMATION

Spectral line formation is a non-Markovian process because the reemiss ion spectrum of an atom depends on the frequency of the photon previou sly absorbed. We account for this effect by adopting a picture that as cribes finite widths to the atomic energy levels. History and success of this atomic model are discussed in some detail. In our semiclassica l model, the state of an atom is characterized not only by its velocit y and its excitation state, but also by the ''substate'' of the broade ned atomic level it occupies. Accordingly, we define a generalized dis tribution function of the atoms in a given energy level which specifie s their number density, their velocity distribution, and their distrib ution over the substates of the level. In this way, the formation of u npolarized spectral lines by multilevel atoms can be described by a se t of kinetic equations for the generalized distribution functions of t he atoms in the various energy levels and a set of transfer equations for the radiation intensities in the various spectral lines. The expli cit form of these equations is derived taking into account all relevan t radiative and collisional processes. The two-level atom is considere d in detail, with special emphasis on the structure of the emission co efficient which is composed of three parts corresponding to complete r edistribution in the laboratory frame, complete redistribution in the atomic rest frame, and coherent reemission in the atomic rest frame, r espectively. (C) 1994 Academic Press, Inc.