RELATIVISTIC MANY-BODY EFFECTS IN THE FINE AND HYPERFINE-STRUCTURE OFLA-139-II (5D- THE NEED FOR 2ND-ORDER ELECTROSTATIC CORRECTIONS(6S)(2) J=2 STATES )

Using our relativistic-configuration-interaction methodology, we repor t multireference calculations of energy differences and magnetic-dipol e and electric-quadrupole hyperfine structure constants of all five le vels in the (5d+6s)(2) J=2 manifold of singly ionized La-139. The erro rs in the splittings of the bottom four levels are no more than 0.02 e V, while the separation of the two highest levels has an error of 0.11 eV due to an incomplete description of the correlation in the levels belonging to higher manifolds. The hyperfine-structure constants show strong sensitivity to many-body effects, resulting in striking improve ments over the multiconfiguration Dirac-Fock values. Seven of the ten hyperfine constants are accurate within 2-12 %. For the rest of the co nstants we have identified the primary cause of the errors as an incor rect description of many-body effects in the high-lying levels and hav e discussed possible remedies.