High-accuracy Measurements of Core-excited Transitions in Light Li-like Ions

TL;DR

This study precisely measured core-excited transition energies in light Li-like ions, significantly reducing uncertainties and improving the accuracy of astrophysical velocity measurements.

Contribution

It introduces an improved calibration method addressing monochromator fluctuations, achieving 5 ppm energy accuracy for core-excited transitions in Li-like ions.

Findings

Achieved 5 ppm energy measurement accuracy.

Reduced velocity uncertainty to 1.6 km/s for astrophysical applications.

Demonstrated fair agreement with theoretical predictions.

Abstract

The transition energies of the two $1s$-core-excited soft X-ray lines (dubbed q and r) from $1s^2 2s ^1S_{1/2}$ 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.