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

Abstract

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 $6p^2$ and six terms of $5d^{9}6s^{2}6p$ have been identified. Evidence of perturbation of $6s5f$ and $6s7p$ by $5d^{9}6s^{2}6p$ and of $6s6d$ by $6p^2$ is presented. The perturbation of $6s6d$ by $6p^2$ in Bi IV and other members of the Hg I sequence is shown to agree qualitatively with Bacher's theory of $sd-p^2$ 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 $6s7p_{\frac{1}{2}}$. This is interpreted as a coupling effect due to the approximate equality of the multiplet and hyperfine structure separations of $6s7p_{\frac{1}{2}}$. Goudsmit and Bacher's theory of hyperfine structure perturbations due to a change in coupling is applied to the four hyperfine levels of $6s7p_{\frac{1}{2}}$. Their theory accounts for the observed distribution of these levels and for the appearance of forbidden transitions.