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 1s 2s^2 2p^2 J - 1s^2 2s^2 2p J' lines is ≈ 3.8 ppm for sulfur and ≈ 2.5 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 meV . The theoretical calculations have been extended to B, C^+ , Si^9+ , Cr^19+ and Fe^21+ , which were the only B-like ions where such transitions were measured up to now.