High-accuracy measurements of core-excited transitions in light Li-like ions

Abstract

The transition energies of the two 1s core-excited soft x-ray lines (dubbed q and r) from 1s22s1S1/2 to the respective upper levels 1s(2S)2s2p(3P)2P3/2 and 2P1/2 of Li-like oxygen, fluorine, and neon were measured and calibrated using several nearby transitions of He-like ions. The major remaining source of energy uncertainties in monochromators, the periodic fluctuations produced by imperfect angular encoder calibration, is addressed by a simultaneously running photoelectron spectroscopy measurement. This leads to an improved energy determination of 5 parts per million, showing fair agreement with previous theories as well as with our own, involving a complete treatment of the autoionizing states studied here. Our experimental results translate to an uncertainty of only 1.6 km/s for the oxygen line qr blend used to determine the outflow velocities of active galactic nuclei, ten times smaller than previously possible. Published by the American Physical Society 2024