Forbidden optical transition in Ti-like Xe, Ba, and Ir

We present measurements of the (3d4)5D2-5D3 transitions in the Ti-like ions Xe32+, Ba34+, and Ir55+ produced and trapped in the Heidelberg electron beam ion trap. The obtained wavelengths have a precision at the few ppm-level and are thereby the most precise measurements of these transitions up to date. For Z=60-75 semi-empirical calculations have shown excellent agreement, however our measurements combined with data from other works shows that outside this range predictions quickly deviate. The value obtained for Ir55+ 357.434(2) nm confirms the linear mismatch to ab initio calculations for Z > 70, as hypothesized in Utter et al., Phys. Rev. A 67, 012508 (2003).