Photoabsorption in hot, dense plasmas - The average atom, the spherical cell model, and the random phase approximation

In this report we show that the orbitals calculated in the average atom (AA) or in the spherical cell (SC) models cannot represent an "average excited orbital" of an atom or ion in a plasma and therefore they give poor approximations for photoabsorption cross sections. We propose here the use of the random phase approximation (RPA) for inhomogeneous systems at finite temperature for the calculation of cross sections for photoabsorption from the initial equilibrium state. The essential feature of the RPA that is most important for the present purpose is that it produces the appropriate potential for the excited electron and incorporates channel-coupling effects. We also suggest the use of the Matsubara Green's function technique which we show can be used for the derivation of the RPA and which also easily lends itself to the definition of higher approximation schemes for systems in equilibrium at finite temperature with attendant physical interpretation. (C) 1997 Elsevier Science Ltd. All rights reserved.