Energy differences of the type 4ƒN-15d–4ƒN (doubly ionized atoms), 4ƒN-15d6s–4ƒN6s (singly ionized atoms), and 4ƒN-15d6s2–4ƒN6s2 (neutral atoms) are important for the interpretation of rare-earth spectra. Here N = 1 for La, … 14 for Yb, and each configuration is represented by its lowest level in these system differences (SD). Nineteen of these differences are now known exactly from analyses of the spectra. Five additional SD values are approximately fixed by a combination of observation and calculation. Regularities deduced from these known values allow predictions for the remaining 18 SD values. The estimated probable error for most of the predictions is either 1000 or 1500 cm-1.
W. C. MARTIN, "Energy Differences between Two Spectroscopic Systems in Neutral, Singly Ionized, and Doubly Ionized Lanthanide Atoms," J. Opt. Soc. Am. 61, 1682-1686 (1971)