High-precision,reference-free measurements of 2p → 1s transitions in boron-like sulfur and argon

We have measured several 2p -> 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 1s2s(2)2p(2) J-1s(2)2s(2)2p J ' lines is approximate to 3.8ppm for sulfur and approximate to 2.5ppm 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 49meV. The theoretical calculations have been extended to B, C+, Si9+, Cr19+ and Fe21+, which were the only B-like ions where such transitions were measured up to now.