Precision measurement of the 3s1/2-3p3/2transition energy in Na-like platinum ions

We report a measurement of the 3${\mathit{s}}_{1/2\mathrm{-}}$3${\mathit{p}}_{3/2}$ transition energy in Na-like ${}_{78}{}^{}{}_{}{}^{}\mathrm{Pt}_{}^{67+}{}_{}{}^{}$ ions of 653.44±0.02(stat) ±0.05(syst) eV. The x rays were observed from ions in an electron-beam ion trap. The uncertainty in our result corresponds to 1% of the total estimated quantum electrodynamic radiative contribution to this transition energy. This value differs significantly from extrapolations based on previous lower-Z data, and establishes a benchmark for calculations of high-Z multielectron radiative and relativistic effects.