Optical transitions in highly-charged californium ions with high   sensitivity to variation of the fine-structure constant
  We study electronic transitions in highly-charged Cf ions that are within the frequency range of optical lasers and have very high sensitivity to potential variations in the fine-structure constant, alpha. The transitions are in the optical despite the large ionisation energies because they lie on the level-crossing of the 5f and 6p valence orbitals in the thallium isoelectronic sequence. Cf16+ is a particularly rich ion, having several narrow lines with properties that minimize certain systematic effects. Cf16+ has very large nuclear charge and large ionisation energy, resulting in the largest alpha-sensitivity seen in atomic systems. The lines include positive and negative shifters. 
J. C. Berengut, V. A. Dzuba, V. V. Flambaum, and A. Ong
physics.atom-ph