the outgoing channel: the ion, the colliding electron, and at least two ejected electrons. The interactions among all these particles via the Coulomb potential must be accounted for [@Berakdar:PhysLett:1996; @Gotz:JPhysB:2006]. Thus, most multiple-ionization cross sections are from experimental measurements or semiempirical formulae derived from fits to experimental data. To calculate the CSD necessary for astrophysics, data are needed for essentially all the charge states of all the elements from H–Zn. Given practical limitations, experimental studies cannot generate all these required data.

For a few systems, however, there now exist sufficient empirical data to incorporate multiple ionization into CSD calculations. Here we focus on iron, which is a cosmically abundant element that forms many emission lines commonly used in astrophysical spectroscopy. We have used semiempirical formulae calibrated to experimental data to derive multiple-ionization cross sections for the various charge states of iron. These cross sections were then incorporated into CSD calculations for both equilibrium and dynamic plasmas. Our results confirm that for equilibrium plasmas, multiple ionization has little effect on the charge balance, modifying the ion abundances by at most about 5%. Conversely, in evolving plasmas, the effects can be significant.

The rest of this paper is organized as follows. In 