Recombination Spectra of Helium‐like Ions

Abstract

We calculate the recombination spectra of the He-like ions He I, C V, N VI, O VII, Ne IX, Mg XI, Si XIII, S XV, Ar XVII, Ca XIX, and Fe XXV. We include the following physical processes: radiative recombination, dielectronic recombination, three-body recombination, electron impact ionization, and collisional excitation by electrons, protons, and α-particles. The calculations also account for the effects of lowering of the continuum at high densities and high-density corrections to dielectronic recombination. From the populations of all levels in the recombined ions, we construct models for He-like ions for fast computation of their spectra. Every model includes 29 bound levels up to n = 5, a pair of superlevels that account for radiative and collisional cascades from highly excited levels, six doubly excited levels that account for the most important satellite lines, and a level that represents the hydrogenic recombining ion. The models are constructed in a way that allows for the correct approach to LTE under appropriate conditions. These models can simultaneously solve for the H/He-like ionization balance in photoionized or collisionally ionized plasmas and compute emission spectra, including the combined effects of radiative and dielectronic recombination, collisional excitation, photoionization from excited levels, fluorescence, and line trapping. The models can be used for any temperature between 100 and 109 K and electron densities of up to 1018 cm-3. The models can be used easily within spectral modeling codes or as stand-alone tools for spectral analysis. We present comparisons between the results of the present models and previous work. Significant differences are found between the present effective recombination rate coefficients to the n = 2 and those of previous estimates. Later, we study various emission-line ratio diagnostics under collisional ionization and photoionized conditions.