High-precision measurements of $n=2to n=1$ transition energies and level widths in He- and Be-like Argon Ions

We performed a reference-free measurement of the transition energies of the $1s 2p,^1P_1to 1s^2 ,^1S_0$ line in He-like argon, and of the $1s 2s^2 2p,^1P_1to 1s^2 2s^2,^1S_0$ line in Be-like argon ions. The highly-charged ions were produced in the plasma of an Electron-Cyclotron Resonance Ion Source. Both energy measurements were performed with an accuracy better than 3 parts in $10^6$, using a double flat-crystal spectrometer, without reference to any theoretical or experimental energy. The $1s 2s^2 2p,^1P_1to 1s^2 2s^2,^1S_0$ transition measurement is the first reference-free measurement for this core-excited transition. The $1s 2p,^1P_1to 1s^2 ,^1S_0$ transition measurement confirms recent measurement performed at the Heidelberg Electron-Beam Ion Trap (EBIT). The width measurement in the He-like transition provides test of a purely radiative decay calculations. In the case of the Be-like argon transition, the width results from the sum of a radiative channel and three main Auger channels. We also performed Multiconfiguration Dirac-Fock (MCDF) calculations of transition energies and rates and have done an extensive comparison with theory and other experimental data. For both measurements reported here, we find agreement with the most recent theoretical calculations within the combined theoretical and experimental uncertainties.