Fine structure splitting of the 3p_1/2-3p_3/2 levels of Na-like Ir⁶⁶⁺

Quantum electrodynamics (QED) is currently one of the most precise theories governing fundamental interactions. Therefore, conducting stringent experimental tests of QED remains crucial. Highly charged quasi-hydrogenic ions like Na-like Ir⁶⁶⁺ are simple systems for testing QED and probing relativistic effects in atomic systems [3]. The fine structure splitting of the 3p_1/2-3p_3/2 transition in Na-like Ir⁶⁶⁺ is a purely relativistic effect; the separation grows with Z and is about 450 eV for Na-like Ir⁶⁶⁺. We used the NIST Electron Beam Ion Trap (EBIT) to create and trap the Ir ions and measured the emitted photons using a superconducting transition-edge sensor array (TES) [1] in the x-ray and a grazing incidence grating spectrometer [2] in the extreme ultraviolet (EUV) spectral ranges. X-ray spectra were calibrated over a 250 eV range using nearby, precisely known lines of H-like and He-like O and N. EUV calibration was based on well-known lines of highly charged Ne, Ba, and Xe. Fine-structure splitting of the 3p_1/2-3p_3/2 levels was determined and compared with relativistic many-body perturbation theory (RMBPT) [3] and multiconfiguration Dirac-Fock calculations [4]. With a projected uncertainty of 70 meV, our measurements provide one of the most precise fine structure separation measurements of Na-like ions in the high-Z atomic number region.