A new benchmark of soft X-ray transition energies of Ne, CO$_2$, and SF$_6$: paving a pathway towards ppm accuracy

TL;DR

This study establishes a high-precision benchmark for soft X-ray transition energies in Ne, CO$_2$, and SF$_6$, aiming to achieve ppm accuracy for spectral interpretation by combining experimental measurements with ab initio calculations.

Contribution

It introduces a novel calibration approach for X-ray transition energies that improves accuracy and discusses the limitations and potential of reaching 1-10 meV uncertainty levels.

Findings

CO$_2$ results align with previous data

SF$_6$ spectrum shifted by ~0.5 eV from earlier results

Ne results differ significantly from past measurements

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 Ne, CO$_2$, and SF$_6$ 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 CO$_2$ result agrees well with previous measurements, the SF$_6$ spectrum appears shifted by ~0.5 eV, about twice the uncertainty of the earlier results. Our result for Ne 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.