Electron-impact excitation of Ni II

Abstract

Aims. Energy levels, transition probabilities, and oscillator strengths are calculated for the second most abundant iron peak element Ni II. The difficulty in obtaining an accurate target representation is related to the open d-shell nature of the target, which has a minimum requirement of single and double promotions from the ground state configuration to the n = 4 shells. Therefore, in order to achieve an accurate representation of the target ion, we have also included configurations containing the 4d, 5s, and 5p subshells. We have undertaken a study of the electron impact excitation of Ni II and present here the collision strengths for forbidden and allowed transitions among the lowest 800 fine-structure levels as well as the corresponding Maxwellian-averaged effective collision strengths for a range of astrophysically relevant electron temperatures. Methods. An accurate Ni II target structure was generated using the modified General-purpose Relativistic Atomic Structure Package (GRASP0) for the lowest lying 1220 jj fine-structure levels, comprising the 11 configurations: 3p63d9, 3p63d84s, 3p63d84p, 3p63d84d, 3p63d85s, 3p63d85p, 3p63d74s2, 3p63d75s2, 3p63d74s4p, 3p63d74s4d, and 3p43d94s4d. The relativistic parallel Dirac atomic R-matrix codes (DARC) were utilised in the scattering calculations to generate the collision strengths for incident electron energies between 0 and 2 Ryd and, by employing infinite dipole and non-dipole limit points, we also generated the effective collision strengths for temperatures in the range from 1000 to 400 000 K. Two separate calculations were performed, both comprised of truncated close-coupling expansions of 800 jj-levels with the first calculation retaining the theoretical ab initio energy levels generated in the GRASP0 evaluations, whereas in the second calculation these energies were shifted to their predicted National Institute of Standards and Technology (NIST) values where possible. This should provide a lower estimate on the uncertainty. Results. Comparisons are made between the radiative data and the collisional cross sections with past theoretical and experimental studies. The effective collision strengths when compared with the most recent published calculations, are found to agree to within 10% for the majority of the transitions considered. In addition, the data are used to model the spectrum of Ni II and good agreement is found with previous investigations and observations.