Spin-orbit interaction in the first excited 4f-level in trivalent lanthanides

The spin-orbit interaction has been considered for the first excited 4flevel in free ions Ln IV and Ln3+: LaF3 crystals. The matrix elements of spin-orbit interaction have been calculated and varied with the level population at T = 298.15 K. It has been found that the first excited 4flevels in these systems exhibit lower values of matrix elements than those for the ground levels, except Yb IV, Yb3+: LaF3. The ratio of the spin-orbit coupling constants zeta 4f (crystal)/zeta 4f (free ion) is <= 1 in the series, thus revising the concept for nephelauxetic effect in this relation.