Calculated oscillator strengths and transition probabilities of singly ionised nickel (Ni II)

¹ Physics Department, Imperial College London, London SW7 2AZ, UK
² Anton Pannekoek Institute for Astronomy (API), University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³ Space Research Organisation Netherlands (SRON), Niels Bohrweg 4, 2333 CA Leiden, The Netherlands

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 September 2025
Accepted: 25 December 2025

Abstract

Aims. This work reports calculated transition probabilities for spectral lines of singly ionised nickel (Ni II) incorporating newly determined experimental energy levels, addressing critical gaps in atomic data required for astrophysical spectroscopy and plasma diagnostics.

Methods. Transition probabilities of Ni II were calculated using the semi-empirical orthogonal operator method for both odd and even energy levels. Calculated eigenvalues were fine-tuned to experimental energy levels, determined using Fourier transform spectroscopy, further increasing the accuracy of these calculated transition probabilities.

Results. In total, transition probabilities have been calculated for nearly 118 000 electric dipole transitions between 361 even and 735 odd levels. The resulting transition probabilities show strong agreement with existing experimental and semi-empirical data, while offering improved consistency and coverage across a wide range of line strengths. The calculated transitions span the far-infrared to the vacuum ultraviolet spectral regions, providing extensive coverage for astrophysical applications. This dataset significantly enhances the calculated atomic data available for Ni II and represents a critical contribution to the advancement of our understanding of astrophysical phenomena through improved spectroscopic analysis.