A new benchmark of soft X-ray transition energies of , , and : paving a pathway towards ppm accuracy

A key requirement for the correct interpretation of high-resolution X-ray spectra is that transition energies are known with high accuracy and precision. We investigate the K-shell features of $$ , $$ , and $$ gases, by measuring their photo ion-yield spectra at the BESSY II synchrotron facility simultaneously with the 1s-np fluorescence emission of He-like ions produced in the Polar-X EBIT. Accurate ab initio calculations of transitions in these ions provide the basis of the calibration. While the $$ result agrees well with previous measurements, the $$ spectrum appears shifted by $$ 0.5 eV, about twice the uncertainty of the earlier results. Our result for $$ shows a large departure from earlier results, but may suffer from larger systematic effects than our other measurements. The molecular spectra agree well with our results of time-dependent density functional theory. We find that the statistical uncertainty allows calibrations in the desired range of 1-10 meV, however, systematic contributions still limit the uncertainty to $$ 40-100 meV, mainly due to the temporal stability of the monochromator energy scale. Combining our absolute calibration technique with a relative energy calibration technique such as photoelectron energy spectroscopy will be necessary to realize its full potential of achieving uncertainties as low as 1-10 meV.