Spectroscopy of highly charged ions near the 4f-5s level crossing

Optical and ultraviolet transitions in highly charged ions are uncommon due to the energy scaling of energy levels with atomic number Z2. However, at level crossings such as the 4f-5s crossing, multiple-electron configurations are nearly degenerate, and thus many optical transitions can exist. The complex many-electron couplings reduce the accuracy of current calculations. Moreover, experimental data to benchmark the predictions is lacking. This is unfortunate, because these ions have been shown to have advantageous properties for new atomic clocks and for searches of variation of the fine-structure constant. Furthermore,ultraviolet and optical transitions are highly suitable for plasma diagnostic purposes. At the Heidelberg electron beam ion trap we have performed spectroscopy in the optical and ultraviolet range of ions with level structures near the 4f-5s level crossing. The obtained results pave the way for future precision laser spectroscopy experiments with the aim of building clocks reaching unprecedented precision.