Dielectronic recombination rate coefficients for highly ionized Ni-like atoms

Ab initio calculations of the total dielectronic recombination (DR) rate coefficients far ten ions along the Ni I isoelectronic sequence in the ground state (Mo14+, Ag19+, Xe26+, Pr31+, Gd36+, Dy38+, Ta45+, Au51+, Al57+, and U64+) have been performed using the HULLAC computer code package. Resonant and nonresonant stabilizing radiative transitions were included. Collisional transitions following electron capture were neglected, The present level-by-level calculations include the contributions of all the levels (over 17 000) belonging to the following Cu-like inner-shell excited configurations: 3d(9)4ln'l' (n'less than or equal to 9), 3p(5)3d(10)4ln'l' (n'less than or equal to 5), and 3s3p(6)3d(10)4ln'l' (n'less than or equal to 5). The configuration complexes with a hole in the 3p inner shell contribute about 10% to the total DR rate coefficients and the complexes with the hole in the 3s inner shell about 1%. The contributions of 3d(9)4ln'l' for n'>9 were evaluated by extrapolation, applying an n(-3) scaling, which was checked fur the specific Ta45+ case. it is shown that at electron temperatures higher than half the ionization energy E(l)(Cu) of the Cu-like ion, the Burgess-Merts (BM) semiempirical formula can provide DR results with an accuracy better than +/-20% for the relatively heavy ions (Z>54), whereas-for the lighter ions it leads to an underestimation of up to a factor 2 (fur Mo). Oil the other hand, al low electron temperature: [kT(e)<0.3E(l)(Cu)] the BM approximation underestimates the DR rate coefficients by up to a few orders of magnitude and its temperature dependence is completely inadequate.