A theoretical study on trivalent europium: from the free ion to the water complex.

The energy levels within the (7)F and (5)D manifolds of trivalent europium were computed for the free ion and in the crystal field of 6-9 water molecules. Fully relativistic Kramers pairs configuration interaction (KRCI) calculations were performed with different correlation spaces for the free ion, and with the complete open-shell configuration interaction (COSCI) method including the 4f electrons in the active space for the water clusters. The best agreement with experimental data was found with the KRCI method or when including the spin-other-orbit effect in the COSCI calculations. For the free ion, the (7)F6 multiplet is found only 162 cm(-1) higher compared to experiment, while the (5)D(0-3) multiplets are approximately 3100 cm(-1) too high. In the crystal field of six water molecules, the multiplets with J > 0 split by 48-123 cm(-1). The energy separation (7)F0-(5)D0 of the unsplit multiplets is computed within 255 cm(-1)/247 cm(-1) compared to the experimental data for the free ion/in water. It has been found that the effect of higher coordination number or lower symmetry is small, increasing the transition energies by only about 40 cm(-1) by lowering the (7)F(J) states by the same amount. The hypersensitive transition (5)D0-(7)F2 is computed at 17079 cm(-1) with the KRCI method as compared to the experimental value of 16267 cm(-1).