Identification of optical transitions in complex highly charged ions for applications in metrology and tests of fundamental constants

Forbidden optical transitions in highly charged ions have become subject of extensive theoretical investigations due to their suitability as next generation frequency standards and for tests on a possible variation of the fine structure constant alpha. In particular, a level crossing in the Nd-like ion Ir¹⁷⁺ provides optical transitions with the highest sensitivity to such a variation ever predicted for a stable atomic system. However, the vast majority of the proposed ions have never been investigated experimentally, and accurate theoretical calculations are difficult due to complex electron correlations. This work explores the optical spectra of Nd-like W, Re, Os, Ir, and Pt with the Heidelberg electron beam ion trap. A method has been developed to identify fine structure transitions in isoelectronic sequences by exploiting the scaling of transition energies depending on the atomic number Z. This method was independently validated by the analysis of the Zeeman splitting in high resolution and accuracy spectra of the brightest Ir¹⁷⁺ lines. Advanced atomic structure calculations could be experimentally tested for the first time in this class of complex ions providing a benchmark for future calculations. From the achieved identifications, the transition energies of proposed frequency standards in Hf¹²⁺ and W¹⁴⁺ were inferred, as well as possible values for the sought-after alpha-sensitive M2/E3 transition energies in Ir<sup>17+