Storage Ring Cross Section Measurements for Electron Impact Single and Double Ionization of Fe^13+ and Single Ionization of Fe^16+ and Fe^17+

TL;DR

This study measures electron impact ionization cross sections for Fe ions using an ion storage ring, revealing discrepancies with previous experiments and theories, and providing new, more accurate data for specific ionization processes.

Contribution

First measurements of EII for Fe^13+, Fe^16+, and Fe^17+ using an ion storage ring, with improved accuracy and comparison to existing theories and experiments.

Findings

Single ionization of Fe^13+ is over 40% lower than previous experiments.

Theoretical models by Arnaud & Raymond and Dere overestimate cross sections.

Results for Fe^16+ and Fe^17+ agree with theoretical predictions within uncertainties.

Abstract

We report measurements of electron impact ionization (EII) for Fe^13+, Fe^16+, and Fe^17+ over collision energies from below threshold to above 3000 eV. The ions were recirculated using an ion storage ring. Data were collected after a sufficiently long time that essentially all the ions had relaxed radiatively to their ground state before data were collected. For single ionization of $\fethirteen$ we find that previous single pass experiments are more than 40% larger than our results. Compared to our work, the theoretical cross section recommended by Arnaud & Raymond (1992) is more than 30% larger, while that of Dere (2007) is about 20% greater. Much of the discrepancy with Dere (2007) is due to the theory overestimating the contribution of excitation-autoionization via n=2 excitations. Double ionization of Fe^13+ is dominated by direct ionization of an inner shell electron accompanied by autoionization of a second electron. Our results for single ionization of Fe^16+ and Fe^17+ agree with theoretical calculations to within the experimental uncertainties.