Eight low-lying levels of Er ii 4f12 6s 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 4f12; the spacings between the two components of the pairs depend on the Coulomb interaction between the s electron and core and are determined by the single Slater integral G3(4f,6s). The levels of the core can be fitted very closely with integrals Fk(4f,4f) whose ratios one to another are the same as those for a hydrogenic 4f eigenfunction. The integral F2(4f,4f) is found to be 427 cm−1 and ζf the spin-orbit coupling constant, 2237 cm−1. The spacings are fitted moderately well by a value of 1580 cm−1 for G3; discrepancies are ascribed to configuration interaction. The g values of the levels are found to agree closely with those calculated on the basis of perfect Jj coupling.
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