Hyperfine structure of the (5d+6s)3configuration ofi139: New measurements andab initiomulticonfigurational Dirac-Fock calculations

The atomic-beam laser-rf double-resonance method has been used to measure precisely the dipole and quadrupole hyperfine structure (hfs) of 11 levels of the 5$d^2$6s configuration and four levels of the 5$d^3$ configuration of ${}_{}{}^{139}{}_{}{}^{}$La i. The results, together with earlier results for lower-lying levels, are compared in detail with new multiconfiguration Dirac-Fock (MCDF) ab initio calculations. The agreement is good to fair overall, but is poor in some areas. The comparison yields new insights and suggests areas in which the theoretical approach must be improved. In particular, the theory underestimates the importance of contact hfs in the 5$d^2$6s configuration by 25–40 %. In addition, there is at present no self-consistent way in the MCDF approach to take account of the large core polarization observed in the 5$d^3$ ${\mathrm{}}^4$F term.