Absolute Wavelength Measurements of Transitions in H- and He-like Argon and Sulfur Ions with a Novel Crystal Spectrometer

High-precision absolute and relative wavelength measurements of highly charged H-like and He-like ions have been carried out at the Heidelberg Electron Beam Ion Trap (EBIT) with a novel crystal spectrometer applying the Bond method. The Ar$^{16+}$ results, with error bars of $\delta\lambda / \lambda < 5 \cdot 10^{-6}$, are the most precise absolute wavelength measurements in highly charged ions up to now and confirm recent relativistic and QED calculations for this range of $Z$. This level of accuracy was reached by introducing a new method for the determination of the Bragg angle using two laser beams as fiducials. These make the commonly-used entrance slits unnecessary, thus reducing the time necessary for reaching high statistical significance, and also eliminating various systematic geometric error sources. A comparison to theory and other experimental results will be presented.