A study of the energy levels of isoelectronic sequences has led to a simple empirical relation to describe these levels. This relation is based on the idea that one can always linearize the ratio E/Zs for some value of s which is nonintegral. Application of this relation to various sequences leads to energy levels which deviate by less than 1% from the experimental values. As a result of the application of this method to the ground term of various sequences, certain coronal lines can be identified as forbidden transitions in highly ionized species. Edlén’s identification of the coronal line at 4086.3 Å as the forbidden 3P1-3P2 transitions of Ca XIII is confirmed. The coronal line at 3454.3 Å can be identified as the forbidden transition 2P3/2-2P1/2 of Cu XIII.
R. AKERIB, "Empirical Relations for the Energy Levels of Atoms and Ions in an Isoelectronic Sequence," J. Opt. Soc. Am. 53, 918-920 (1963)