Dielectronic recombination rate coefficients for highly ionized Ni-like atoms

Ab initio calculations of the total dielectronic recombination (DR) rate coefficients for ten ions along the Ni I isoelectronic sequence in the ground state ([Formula Presented], [Formula Presented], [Formula Presented], [Formula Presented], [Formula Presented], [Formula Presented], [Formula Presented], [Formula Presented], [Formula Presented], and [Formula Presented]) 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: 3[Formula Presented]4ln′l′ (n′≤9), 3[Formula Presented]3[Formula Presented]4ln′l′ (n′≤5), and 3s3[Formula Presented]3[Formula Presented]4ln′l′ (n′≤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 3[Formula Presented]4ln′l′ for n′≳9 were evaluated by extrapolation, applying an [Formula Presented] scaling, which was checked for the specific [Formula Presented] case. It is shown that at electron temperatures higher than half the ionization energy [Formula Presented](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 (for Mo). On the other hand, at low electron temperature [[Formula Presented]<0.3[Formula Presented](Cu)] the BM approximation underestimates the DR rate coefficients by up to a few orders of magnitude and its temperature dependence is completely inadequate.