We report measurements of the $D_1$ and $D_2$ transition energies in Na-like Ir (${\mathrm{Ir}}^{66+}$). The $D_{1}3s\text{−}3p_{1/2}$ transition energy, measured in the extreme ultraviolet (EUV) region at $169.977\pm 0.005$ eV with a relative uncertainty of 29 ppm, represents one of the most precise $D$-line energy measurements to date for high atomic numbers $Z\ge 30$. The $D_{2}3s\text{−}3p_{3/2}$ transition energy, measured in the x-ray region, is $621.11\pm 0.06$ eV. Therefore, there is a fine-structure separation of $451.13\pm 0.06$ eV for the Na-like Ir $3p$ levels. We present theoretical calculations of these energies using quantum electrodynamic-extended relativistic many-body perturbation theory and multiple-configuration Dirac-Hartree-Fock methods, and discuss the discrepancies between experimental and theoretical values. The prospect of determining the absolute nuclear charge radii of heavy elements, including rare isotopes, from these measurements is explored.

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• Atomic spectra
• Electronic transitions
• Fine & hyperfine structure