Absolute UV and Vacuum UV Oscillator Strengths for Niii

Abstract

Radiative lifetimes, measured using time-resolved laser-induced fluorescence, are reported for 18 odd-parity levels of Ni+. The radiative lifetimes are combined with Ni II emission branching fractions to produce 59 accurate (typically ±10%) absolute atomic transition probabilities in the ultraviolet (UV) and vacuum ultraviolet (VUV). The UV and VUV branching fractions are determined using a combination of measurements from a Fourier transform spectrometer at the University of Lund and a 3 m focal length vacuum echelle spectrometer system at the University of Wisconsin. Accurate f-values for Ni II resonance lines are important for analysis of spectra on the diffuse interstellar medium (ISM) at low redshift and for studies of the same lines in high-redshift QSO absorption line systems. Resonance lines at vacuum wavelengths of 170.960, 174.155, and 175.191 nm have log gf values of -0.68, -0.60, and -0.80, respectively. Until additional laboratory measurements are completed, we recommend thatf-values for other Ni II resonance lines be taken from the relative f-values from Zsargo & Federman with a multiplicative scaling factor of 0.534 (±10%). The present measurements indicate that nickel is less depleted in the ISM than previously estimated.