Theoretical study of ground-state configurations 4fN in Nd IV, Pr IV and Nd V

International audienceAlthough the trivalent lanthanide ions embedded in compounds have a wealth of applications in laser physics and the lighting industry, the theoretical interpretation of their ground-state configurations 4fN in the free ion spectra was restricted by the lack of experimental data. In the first part of this paper, the present status of their group-theoretical description and prospects for improvement are given. In the second part, this description is applied for the first time to a truly isolated configuration 4f3 in Nd IV by performing a parametric fit of level energies, taking into account Coulomb and spin-dependent interactions beyond the first order of perturbation. A root mean square (rms) deviation 5.39 cm-1 (i.e. 1/13 000 of the energy spread of the configuration) is obtained with 23 free parameters and 41 newly determined levels. The contributions of all parameters to the level energies are detailed for the first time. It is shown that energy shifts caused by the effective electrostatic parameters related to the third order of perturbation amount to about 10-3 of the total energy spread of the configuration 4f3. As concerns spin-dependent operators, the relative contributions of the spin-orbit interaction (SO), of the combined effects of spin-other-orbit and configuration interaction (SOO + CI) and of the spin-spin interaction (SS) are in the ratios 571:12:1. The Marvin integrals derived from the experimental energies are in a very good agreement with Hartree-Fock estimates. The ground-state configurations 4f2 of Pr IV and of the newly analysed spectrum Nd V are studied with the same methods