Dielectronic recombination rate coefficients for the Ni ${\sf I}$ isoelectronic sequence

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Ab initio calculations of the total dielectronic recombination (DR) rate coefficients for thirteen ions along the NiI isoelectronic sequence in the ground state (Kr8+, Mo14+, Ag19+, Sn22+, Xe26+, Nd32+, Gd36+, Yb42+, W46+, Au51+, Pb54+, At57+, and U64+) have been performed using the flexible atomic code. The level-by-level calculations are performed for evaluating the DR contributions through the relevant Cu-like autoionizing inner-shell excited 3l174l′n′′l′′ and 3l175l′n′′l′′ configuration complexes with n′′ ≤15, which are associated with Δn=1 and Δn=2 core-excitations, respectively. The usual (n′′)-3 scaling law is found to be invalid for low-Z ions. A level-by-level extrapolation procedure is employed to obtain the contributions through higher n′′ complexes. The decays to autoionizing levels followed possibly by radiative cascades could enlarge the rates at relatively high temperature by a factor up to about 23%. For the whole isoelectronic ions the contributions from 3s23p63d9 4l′n′′l′′ dominate the total DR rates while the contributions from the 3s23p63d9 5l′n′′l′′ configuration complexes are about 10-20% at relatively high temperature. On the basis of the calculated results, a general analytic formula for the total DR rate coefficients of all the ions with 36≤Z ≤92 along the NiI isoelectronic sequence is constructed. The comparisons of the rates obtained from the general formula with those from the detailed calculations show that the formula is of high precision, generally better than 3% accuracy for electron temperatures kT≥0.1EI, where EI is the ionization energy of the Cu-like ion. The present DR rates at temperature above 1.0EI are larger than the previously published data by a factor above 30%. The commonly used semiempirical formula proposed by Burgess and modified by Merts may overestimate the rates at high temperature by a factor of about 2 for low-Z ions.