Wavelengths and Energy Levels of the Upper Levels of Singly Ionized Nickel (Ni ii) from 3d⁸(³F)5f to 3d⁸(³F)9s

Using high-resolution spectra of Ni ii recorded using Fourier transform (FT) spectroscopy of continuous, nickel-helium hollow cathode discharge sources in the region 143-5555 nm (1800-70,000 cm⁻¹, the analysis of 1016 Ni ii lines confirmed and optimized 206 previously reported energy levels of the (³ F) parent term, from 3d ⁸(³ F)5f to 3d ⁸(³ F)9s, lying between 122,060 and 138,563 cm⁻¹. The uncertainties of these levels have been improved by at least an order of magnitude compared with their previously reported values. With the increased resolution and spectral range of the FT measurements, compared to previously published grating spectra, we were able to extend our analysis to identify and establish 33 new energy levels of Ni ii, which are reported here for the first time. Eigenvector compositions of all revised and newly established energy levels were calculated using the orthogonal operator method. In addition, an improved ionization energy of 146,541.35 ± 0.15 cm⁻¹ for Ni ii, using highly excited levels of the 3d ⁸(³ F)5g, 3d ⁸(³ F)6g, and 3d ⁸(³ F)6h configurations, has been derived.