Eight low-lying levels of Er II 4ƒ<sup>12</sup> 6<i>s</i> have been observed to be grouped according to their energy in four close pairs. The centers of gravity of the pairs correspond to the energies of the four lowest levels of the core 4ƒ<sup>12</sup>; the spacings between the two components of the pairs depend on the Coulomb interaction between the <i>s</i> electron and core and are determined by the single Slater integral <i>G</i><sup>3</sup> (4ƒ,6<i>s</i>). The levels of the core can be fitted very closely with integrals <i>F<sub>k</sub></i> (4ƒ,4ƒ) whose ratios one to another are the same as those for a hydrogenic 4ƒ eigenfunction. The integral <i>F</i><sub>2</sub> (4ƒ,4ƒ) is found to be 427 cm<sup>-1</sup> and ζ<sub>ƒ</sub> the spin-orbit coupling constant, 2237 cm<sup>-1</sup>. The spacings are fitted moderately well by a value of 1580 cm<sup>-1</sup> for <i>G</i><sup>3</sup>; discrepancies are ascribed to configuration interaction. The <i>g</i> values of the levels are found to agree closely with those calculated onthe basis of perfect <i>Jj</i> coupling.
B. R. JUDD and L. C. MARQUET, "Energy Levels of Er II," J. Opt. Soc. Am. 52, 504-506 (1962)