Interpretation of the hyperfine structures in the low even configurations of lanthanum II

The hyperfine structures (hfs) of the levels of the 5d ² and 5d6s configurations of La II, recently measured very accurately by Höhle, Hühnermann and Wagner, are interpreted in the classical parametric scheme. For the magnetic part, the contribution of the two-body far-configuration-mixing effects is determined accurately from the hfs of four levels, and compared with an ab-initio evaluation. For the electric part, all three 5d parameters are determined. The Sternheimer corrections to the radialr ^?3 integral are computed by the Multiconfigurational-Hartree-Fock method, and a corrected value of the electric-quadrupole nuclear moment is deduced:Q(¹³⁹La)=0.20(1) 10^?24 cm².