New Ritz Wavelengths and Transition Probabilities for Parity-Forbidden, Singly Ionised Nickel [Ni ii] Lines of Astrophysical Interest


 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% for strong M1 lines and up to 10% 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.