Modeling of ionic spectra

Abstract

In the case of coalescence of the lines, an unresolved transition array (UTA) can be simply represented, as a Gaussian (or more complicated) curve, using the computed distribution moments. Here, there appears a conspicuous difference between the emission and absorption spectra. The linewidths, and the overlapping of the lines (and the gaps between them) have a critical effect on the value of the total absorption, but not on that of the total emission. The emission spectrum can be represented accurately as a collection of Gaussian curves, one per UTA, but not the transmission spectrum.

If the aim of the calculation is the computation of the Rosselandmean opacity, the transmission lines ought to be represented individually, and the distribution moments can be used in another way. For each line, the wavenumbers of the two levels involved, and the intensity, are picked at random in a triple distribution, in which all three quantities are correlated. This is the model of Resolved Transition Arrays, which yields correct values for the distribution moments of the line energies and intensities. This kind of evaluation is checked by comparison of the corresponding values of the Rosseland mean opacities with those obtained through the classical level-by-level calculations.