The present model is the time-dependent version of a previous model (Sureau et al. 1983) in which the population distribution was assumed in steady state. A finite set of levels is partitioned in four subsets: the Z-ion ground-level and, contingently, the first near-degenerated levels (subset 1); all the successive excited Z-ion levels up to n=5 (subset 2); a finite number of higher Rydberg levels (because of the limitation of the series in the plasmas) which are assumed in LTE with the Z+l−ion ground-level (subset 3, called the thermal band); and the Z+l ion ground-level (subset 4).

The physical processes explicitly considered are the radiative cascades and the transitions between the Z-ion bound levels induced by electron-ion collisions. The radiative-transition probabilities are given by ab-initio calculations using a modified Hartree-Fock method including the spin-orbit interaction (Sureau et al., 1984). The collision rates are derived by the Van Regemorter formula multiplied by an adjustable parameter Fc.