Lifetime Measurements for Ground Term Transitions in Ta ii , W ii , and Re ii

Abstract

Lifetime measurements have been made for the radiation emitted in the ground term transitions of the nominal form 5s 2 5p 6 5d x 6s-5s 2 5p 6 5d x 6p in Ta II (x = 3), W II (x = 4), and Re II (x = 5). Supporting theoretical calculations have also been performed to characterize configuration interaction and to estimate the degree of decay branching of the upper levels. The lifetime measurements have been combined with these theoretical considerations and with branching fractions deduced from the emission measurements of Corliss & Bozman to specify the oscillator strengths of these transitions. The results are discussed in the context of spectroscopic data obtained from orbiting telescopes and applied to Hubble Space Telescope spectra to estimate the tungsten abundance in Sirius.