2023MNRAS.519.4040C

We report accurate Ritz wavelengths for parity-forbidden [Ni II] transitions, derived from energy levels determined using high-resolution Fourier transform spectroscopy. Transitions between the 18 lowest Ni II energy levels of even-parity produced Ritz wavelengths for 126 parity-forbidden lines. Uncertainties for the Ritz wavelengths derived in this work are up to two orders of magnitude lower than previously published values. Transition probabilities were calculated using the semi-empirical orthogonal operator method, with uncertainties ranging from approximately 1 per cent for strong M1 lines and up to 10 per cent for weak E2 lines. Accurate forbidden line wavelengths and transition probabilities, particularly for lines in the infrared, are important in the analyses of low-density astrophysical plasmas, such as supernova remnants, planetary nebulae, and active galactic nuclei.