Multiplet and Hyperfine Structure Analyses of Bi IV. Discussion of Perturbation Effects

Three hundred and sixty lines of Bi IV, excited in an electrodeless discharge, have been classified. Twenty-seven terms of $6snx$ configurations, four terms of 6p² and six terms of 5d⁹6s²6p have been identified. Evidence of perturbation of $6s5f$ and $6s7p$ by 5d⁹6s²6p and of $6s6d$ by 6p² is presented. The perturbation of $6s6d$ by 6p² in Bi IV and other members of the Hg I sequence is shown to agree qualitatively with Bacher's theory of sd-p² interactions. The hyperfine structure intervals of many of the terms of Bi IV have been evaluated. They are compared with the theoretical formulae for hyperfine structure separations and shown to be consistent with our multiplet analysis. Anomalous hyperfine structure has been observed in 6s7p1/2. This is interpreted as a coupling effect due to the approximate equality of the multiplet and hyperfine structure separations of 6s7p1/2. Goudsmit and Bacher's theory of hyperfine structure perturbations due to a change in coupling is applied to the four hyperfine levels of 6s7p1/2. Their theory accounts for the observed distribution of these levels and for the appearance of forbidden transitions.