High-resolution Laboratory Measurements of K-shell X-ray Line Polarization and Excitation Cross Sections in Heliumlike S XV Ions

TL;DR

This study provides high-resolution measurements of K-shell X-ray line polarization and excitation cross sections in heliumlike S XV ions, enhancing understanding of atomic processes relevant to plasma diagnostics.

Contribution

It introduces a novel experimental approach combining polarization measurements with direct cross section normalization for heliumlike ions.

Findings

Measured excitation cross sections with high precision.

Determined polarization degrees of specific X-ray lines.

Identified dominant line-formation mechanisms such as cascades and ionization.

Abstract

We report measurements of electron-impact excitation cross sections for the strong K-shell n=2-1 transitions in S XV using the LLNL EBIT-I electron beam ion trap, two crystal spectrometers, and the EBIT Calorimeter Spectrometer. The cross sections are determined by direct normalization to the well known cross sections of radiative electron capture, measured simultaneously. Using contemporaneous polarization measurements with the two crystal spectrometers, whose dispersion planes are oriented parallel and perpendicular to the electron beam direction, the polarization of the direct excitation line emission is determined, and in turn the isotropic total cross sections are extracted. We further experimentally investigate various line-formation mechanisms, finding that radiative cascades and collisional inner-shell ionization dominate the degree of linear polarization and total line-emission cross sections of the forbidden line $z$.