Direct measurements for the fine-structure splitting of S VIII and Cl IX

Using electron-beam ion traps (EBITs), we have made direct measurements for the fine-structure splitting 2 p⁵ 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., 10 085.40 ±0.23 cm^-1 for S VIII, where the early value observed in the solar spectrum is questionable, and 13 643.39 ±0.32 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 ≤Z ≤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.