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 $1s^{2}2s^1{S}_{1/2}$ to the respective upper levels $1s{(}^{2}S)2s2p{(}^{3}P{)}^2{P}_{3/2}$ and ${}^2{P}_{1/2}$ 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.

7 pages, 4 figures