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 (${\mathrm{Mo}}^{14+}$, ${\mathrm{Ag}}^{19+}$, ${\mathrm{Xe}}^{26+}$, ${\mathrm{Pr}}^{31+}$, ${\mathrm{Gd}}^{36+}$, ${\mathrm{Dy}}^{38+}$, ${\mathrm{Ta}}^{45+}$, ${\mathrm{Au}}^{51+}$, ${\mathrm{At}}^{57+}$, and ${\mathrm{U}}^{64+}$) have been performed using the H U L L A C 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${\mathit{d}}^9$4ln′l′ (n′≤9), 3${\mathit{p}}^5$3${\mathit{d}}^{10}$4ln′l′ (n′≤5), and 3s3${\mathit{p}}^6$3${\mathit{d}}^{10}$4ln′l′ (n′≤5). The configuration complexes with a hole in the 3p inner shell contribute about 10% to the total DR rate coefficients and the complexes with the hole in the 3s inner shell about 1%. The contributions of 3${\mathit{d}}^9$4ln′l′ for n′≳9 were evaluated by extrapolation, applying an ${\mathit{n}}^{\mathrm{-}3}$ scaling, which was checked for the specific ${\mathrm{Ta}}^{45+}$ case. It is shown that at electron temperatures higher than half the ionization energy ${\mathit{E}}_{\mathit{I}}$(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 [${\mathit{kT}}_{\mathit{e}}$${\mathit{E}}_{\mathit{I}}$(Cu)] the BM approximation underestimates the DR rate coefficients by up to a few orders of magnitude and its temperature dependence is completely inadequate. © 1996 The American Physical Society.