A new benchmark of soft X-ray transition energies of [Formula: see text] , [Formula: see text] , and [Formula: see text] : paving a pathway towards ppm accuracy

ABSTRACT: 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 [Formula: see text] , [Formula: see text] , and [Formula: see text] 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 [Formula: see text] result agrees well with previous measurements, the [Formula: see text] spectrum appears shifted by [Formula: see text] 0.5 eV, about twice the uncertainty of the earlier results. Our result for [Formula: see text] 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 [Formula: see text] 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. GRAPHICAL ABSTRACT: [Image: see text]