Storage Ring Cross Section Measurements for Electron Impact Single and Double Ionization of Fe13 + and Single Ionization of Fe16 + and Fe17 +
We report measurements of electron impact ionization for Fe13 +, Fe16 +, and Fe17 + 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. For single ionization of Fe13 +, 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 is more than 30% larger, while that of Dere is about 20% greater. Much of the discrepancy with Dere is due to the theory overestimating the contribution of excitation-autoionization via n = 2 excitations. Double ionization of Fe13 + is dominated by direct ionization of an inner shell electron accompanied by autoionization of a second electron. Our results for single ionization of Fe16 + and Fe17 + agree with theoretical calculations to within the experimental uncertainties.