Hole transitions in multiply-charged ions for precision laser   spectroscopy and searching for alpha-variation
  We consider transitions of electron holes (vacancies in otherwise filled shells of atomic systems) in multiply-charged ions that, due to level-crossing of the holes, have frequencies within the range of optical atomic clocks. Strong E1 transitions provide new options for laser-cooling and trapping, while narrow transitions can be used for high-precision spectroscopy and tests of fundamental physics. We show that hole transitions can have extremely high sensitivity to alpha-variation due to large relativistic corrections, and propose candidate transitions that have much larger alpha-sensitivities than any previously seen in atomic systems. 
J. C. Berengut, V. A. Dzuba, V. V. Flambaum, and A. Ong
physics.atom-ph