IRON Project. XLV. Ar XIII and Fe XXI

The energy levels are identified spectroscopically using a newly developed identification procedure. The procedure also makes a correspondence between the fine structure energy levels and LS terms. This provides the check for completeness of the calculated levels.

Comparison is made of the present energies and the f-values with the available observed and theoretical values. Present transition probabilities agree very well with the relativistic atomic structure calculations of Mendoza et al. for the intercombination transitions, 2s2p^3^(^5^S{deg}_2_) - 2s^2^2p^2^(^3^P_1,2_,^1^D_2_). This further indicates that the importance of the neglected Breit interaction decreases with ion charge and constrains the uncertainty in the present calculations to within 15% even for the weak transitions.