We demonstrate a widely applicable technique to absolutely calibrate the energy scale of x-ray spectra with experimentally well-known and accurately calculable transitions of highly charged ions, allowing us to measure the $K$-shell Rydberg spectrum of molecular ${\mathrm{O}}_2$ with 8 meV uncertainty. We reveal a systematic $\sim 450\mathrm{meV}$ shift from previous literature values, and settle an extraordinary discrepancy between astrophysical and laboratory measurements of neutral atomic oxygen, the latter being calibrated against the aforementioned ${\mathrm{O}}_2$ literature values. Because of the widespread use of such, now deprecated, references, our method impacts on many branches of x-ray absorption spectroscopy. Moreover, it potentially reduces absolute uncertainties there to below the meV level.

• Received 30 March 2020
• Revised 19 October 2020
• Accepted 30 October 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.243001