Direct measurements for the fine-structure splitting of S viii and Cl ix

Abstract

Using electron-beam ion traps (EBITs), we have made direct measurements for the fine-structure splitting $2{p}^{5}$ 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., $10\phantom{\rule{4pt}{0ex}}085.40\ifmmode\pm\else\textpm\fi{}0.23\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ for S viii, where the early value observed in the solar spectrum is questionable, and $13\phantom{\rule{4pt}{0ex}}643.39\ifmmode\pm\else\textpm\fi{}0.32\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ for Cl ix, where only indirect measurements had been performed. High-precision calculations, including ab initio treatment of QED effects, are accomplished for $10\ensuremath{\le}Z\ensuremath{\le}18$ 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.