Experimental and theoretical Ritz–Rydberg analysis of the electronic structure of highly charged ions of lead and bismuth by optical spectroscopy

Abstract

Intra-configuration fine-structure transitions in highly charged ions
(HCI) result in most cases from changes in the coupling of equivalent electrons.
They are multipole forbidden to varying degrees and often occur within the optical
range. In HCI with semi-filled nd and nf subshells, electrons can in principle
couple to states which are energetically close but with very different total angular
momenta, e.g. 0 ≤ J ≤ 10. This gives rise to metastable states with very
long lifetimes that exhibit particularly interesting clock transitions or, as in the
case of orbital level crossings, acquire a considerable sensitivity to a possible
variation of the fine-structure constant α. We investigate both experimentally
and theoretically connecting the ground state and adjacent states of Pb XXII
to Pb XXXIV and Bi X to Bi XV, respectively, covering complex couplings of
electrons in the 4f and 5d shells. These and others such as nd and nf , which
also contain equivalent electrons, are of interest for frequency metrology using
quantum logic spectroscopy, for atomic structure and QED theory tests, and for
the search for Dark Matter candidates. We infer their level structure, benchmark
calculations from two different electronic structure codes, and lay the groundwork
for inferring the wavelengths of forbidden transitions of higher multipolarity in
the optical and VUV region.