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

The transition energies of the two <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>1</a:mn><a:mi>s</a:mi></a:mrow></a:math> core-excited soft x-ray lines (dubbed <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>q</b:mi></b:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mi>r</c:mi></c:math>) from <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mn>1</d:mn><d:msup><d:mi>s</d:mi><d:mn>2</d:mn></d:msup><d:mn>2</d:mn><d:mi>s</d:mi><d:msup><d:mspace width="0.28em"/><d:mn>1</d:mn></d:msup><d:msub><d:mi>S</d:mi><d:mrow><d:mn>1</d:mn><d:mo>/</d:mo><d:mn>2</d:mn></d:mrow></d:msub></d:mrow></d:math> to the respective upper levels <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mn>1</f:mn><f:mi>s</f:mi><f:mrow><f:msup><f:mo>(</f:mo><f:mn>2</f:mn></f:msup><f:mi>S</f:mi><f:mo>)</f:mo></f:mrow><f:mn>2</f:mn><f:mi>s</f:mi><f:mn>2</f:mn><f:mi>p</f:mi><f:mrow><f:msup><f:mo>(</f:mo><f:mn>3</f:mn></f:msup><f:mi>P</f:mi><f:mo>)</f:mo></f:mrow><f:msup><f:mspace width="0.16em"/><f:mn>2</f:mn></f:msup><f:msub><f:mi>P</f:mi><f:mrow><f:mn>3</f:mn><f:mo>/</f:mo><f:mn>2</f:mn></f:mrow></f:msub></f:mrow></f:math> and <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:msup><h:mrow/><h:mn>2</h:mn></h:msup><h:msub><h:mi>P</h:mi><h:mrow><h:mn>1</h:mn><h:mo>/</h:mo><h:mn>2</h:mn></h:mrow></h:msub></h:mrow></h:math> 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 <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mi>q</i:mi><i:mi>r</i:mi></i:mrow></i:math> blend used to determine the outflow velocities of active galactic nuclei, ten times smaller than previously possible. Published by the American Physical Society 2024