Storage Ring Cross Section Measurements for Electron Impact Ionization of Fe 7+

TL;DR

This study provides precise measurements of electron impact ionization cross sections for Fe 7+ ions over a range of energies, highlighting discrepancies with existing theoretical data and emphasizing the importance of accurate atomic modeling.

Contribution

First experimental measurement of Fe 7+ EII cross sections using a storage ring, with detailed comparison to and critique of existing theoretical data.

Findings

Measured cross sections are up to 40% lower than Arnaud & Raymond (1992).

Dere (2007) cross sections agree within 10% of measurements.

Discrepancies likely due to limitations in excitation-autoionization modeling.

Abstract

We have measured electron impact ionization (EII) for Fe 7+ from the ionization threshold up to 1200 eV. The measurements were performed using the TSR heavy ion storage ring. The ions were stored long enough prior to measurement to remove most metastables, resulting in a beam of 94% ground state ions. Comparing with the previously recommended atomic data, we find that the Arnaud & Raymond (1992) cross section is up to about 40\% larger than our measurement, with the largest discrepancies below about 400~eV. The cross section of Dere (2007) agrees to within 10%, which is about the magnitude of the experimental uncertainties. The remaining discrepancies between measurement and the most recent theory are likely due to shortcomings in the theoretical treatment of the excitation-autoionization contribution.