High-accuracy extreme ultraviolet (EUV) spectroscopy of the $1s^{2}2s2p {}^{1}P_1\text{−}1s^{2}2s^2 {}^{1}S_0$ electric dipole ($E1$) transitions in beryllium-like ${\mathrm{S}}^{12+}$ and $\mathrm{C}{\mathrm{l}}^{13+}$ was performed at the Shanghai High-Temperature Superconducting Electron Beam Ion Trap. Utilizing a portable high-resolution soft x-ray and EUV spectrometer, the measured $E1$ transition wavelengths for ${\mathrm{S}}^{12+}$ and $\mathrm{C}{\mathrm{l}}^{13+}$ were determined to be $256.6852\left(53\right)\AA$ and $237.8037\left(62\right)\AA$ (in vacuum), achieving relative uncertainties of 21 ppm and 26 ppm, respectively. Furthermore, the $E1$ transition energies for these ions were evaluated with ultra-high precision within an ab initio quantum electrodynamics (QED) framework to enable a stringent comparison with the experimental data. The measured values are in good agreement with the theoretical predictions, providing a precision test of QED and correlation effects in few-electron highly charged ions.

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• Atomic & molecular structure
• Optical, UV, & IR astronomy
• Ions
• Trapped ions
• Ab initio calculations