Using electron-beam ion traps (EBITs), we have made direct measurements for the fine-structure splitting $2p^5$ of S viii and Cl ix, which are important to the test of ab initio theory in F-like ions. Their wavelength are redetermined in the laboratory with an accuracy of 23 ppm, i.e., $10085.40\pm 0.23{\mathrm{cm}}^{-1}$ for S viii, where the early value observed in the solar spectrum is questionable, and $13643.39\pm 0.32{\mathrm{cm}}^{-1}$ for Cl ix, where only indirect measurements had been performed. High-precision calculations, including ab initio treatment of QED effects, are accomplished for $10\le Z\le 18$ and compared with the experimental results. Good agreement is found between our experimental and theoretical values of the fine-structure energy among these ions except for Al v, which calls for further investigations. Besides, we have succeeded in pushing EBIT operation towards the infrared spectral region relevant to astrophysics.

• Received 12 August 2020
• Accepted 5 October 2020

DOI:https://doi.org/10.1103/PhysRevA.102.042817