Hyperfine-structure studies of Nb ii: Experimental and relativistic configuration-interaction results

We report an experimental and theoretical study of the hyperfine structure (hfs) in various metastable states in ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ i i. Hyperfine structures of five levels in Nb i i have been measured using a combination of the laser-rf double resonance and laser-induced fluorescence methods in a collinear laser–ion-beam geometry. Theoretically, for J=2, a multireference calculation of energies and hfs based on a relativistic configuration-interaction methodology of the lowest ten levels in the (4d+5s${)}^4$ manifold is reported. The average energy error is 450 ${\mathrm{cm}}^{\mathrm{-}1}$. Many of the hyperfine constants show large changes from the Dirac-Fock values and the magnetic dipole constant has a 4% accuracy for the one J=2 level measured. We have also identified all the core-valence and core-core effects that dominate the energy differences and hfs.