High-precision,reference-free measurements of \(2p \rightarrow 1s\) transitions in boron-like sulfur and argon

We have measured several \(2p \rightarrow 1s\) transition energies in core-excited boron-like ions of sulfur and argon. The measurements are reference-free, with an accuracy of a few parts per million. The x-rays were produced by the plasma of an electron cyclotron resonance ion source and measured with a double-crystal x-ray spectrometer. The precision obtained for the measured \(1s 2s^2 2p^2~J - 1s^2 2s^2 2p~J'\) lines is \({\approx 3.8}\,{\hbox {ppm}}\) for sulfur and \({\approx 2.5}\,{\hbox {ppm}}\) for argon. The line energies are compared to relativistic atomic structure calculations performed with the mdfgme multi-configuration Dirac–Fock code. This comparison is used for line identification and tests the theoretical methods, which are in agreement with the experimental data up to \(49\,\hbox {meV}\). The theoretical calculations have been extended to B, \(\hbox {C}^+\), \(\hbox {Si}^{9+}\), \(\hbox {Cr}^{19+}\) and \(\hbox {Fe}^{21+}\), which were the only B-like ions where such transitions were measured up to now.