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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
Z2. At the 4 f –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
4 f –5s level crossing were measured at the Heidelberg electron beam ion trap. A collisionalradiative
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 Re14+, Os15+, Ir16+, and
Pt17+. 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.
