The groups of six levels obtained by adding, in J-j coupling, a 6p electron to the lowest level of the 4ƒN-1 core have been identified in gadolinium, dysprosium, erbiumr, and ytterbium (N = 8, 9, 12, 14). They account for strong transitions to the corresponding 4ƒN-15s6s2 configurations. Values for the spin-orbit parameter of the 6p electron are (in cm-1) ζp = 1835, 1840, 2049, 2257, respectively.
NISSAN SPECTOR, "Configurations 4ƒN-16s26p in Neutral Gadolinium, Dysprosium, Erbium, and Ytterbium," J. Opt. Soc. Am. 61, 1350-1354 (1971)