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Abstract

In recent years, various highly charged ions (HCI) with optical transitions have been proposed for metrology and searches of a possible variation of the fine-structure constant α. Optical transitions in HCI are uncommon due to the scaling of energy levels with atomic number Z^2 . At the 4f–5s level crossing, three 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. Spectra in the optical and extreme-ultraviolet (EUV) range of several ion species near the 4f–5s level crossing were measured at the Heidelberg electron beam ion trap. A collisional-radiative model was employed for the interpretation of the EUV data, resulting in the first identification of the long sought-after 5s–5p transitions in Pm-like Re^14+ , Os^15+ , Ir^16+ , and Pt^17+. The characteristic line shapes of optical transitions in Ir17+ were studied, with the aim of identifying transitions with a high sensitivity to α-variation. Previously suggested candidates could be excluded and new candidates were proposed. This data provides a stringent benchmark for state-of-the-art precision atomic theory.