Spectra of Bi II and Pb II were excited by high-frequency oscillations in a quartz discharge tube conaining the vapor of the metal in an atmosphere of helium. The following five lines, corresponding to transitions among the five lowest levels of Bi II belonging to the 6s2 6p2 electron configuration and forbidden by the Laporte selection rule, were observed: λ3240.7 (1S0→3P1), a pure magnetic dipole line; λ3683.2 (1S0→3P2), a pure electric quadrupole line; two lines of mixed character, λ4850.2 (1D2 → 3P1)and λ5913.5 (1D2→ 3P2 and λ7503.2 (3P1 → 3P0), a pure magnetic dipole line observed for the first time as a nonintercombination transition. In addition, a line was found in the spectrum of Pb II at λ7099.8 (2P½0 → 2P½0). It corresponds to the transition between the two lowest levels of Pb II belonging to the 6s26p electron configuration. This is a line of mixed character representing the first definite identification of a spin reversal line. By use of Fabry-Perot étalons, the hyperfine structure of each of these lines was photographed, establishing the identity of the lines and, at the same time, giving the hitherto unknown splittings of the corresponding levels. The hyperfine structure splitting factors “A” and the quadrupole coupling constants “B” for the levels were calculated from these results and compared with theory. For this purpose, the procedure for the calculation of quadrupole coupling constants as originally given by Casimir was extended to include the case of two equivalent p electrons.
C. DANIEL COLE, "Multipole Lines in the Spectra of Bi II and Pb II," J. Opt. Soc. Am. 54, 859-863 (1964)