STORAGE RING CROSS SECTION MEASUREMENTS FOR ELECTRON IMPACT IONIZATION OF Fe¹¹⁺FORMING Fe¹²⁺AND Fe¹³⁺

Abstract

We report ionization cross section measurements for electron impact single\nionization (EISI) of Fe^11+$ forming Fe^12+ and electron impact double\nionization (EIDI) of Fe^11+ forming Fe^13+. The measurements cover the\ncenter-of-mass energy range from approximately 230 eV to 2300 eV. The\nexperiment was performed using the heavy ion storage ring TSR located at the\nMax-Planck-Institut fur Kernphysik in Heidelberg, Germany. The storage ring\napproach allows nearly all metastable levels to relax to the ground state\nbefore data collection begins. We find that the cross section for single\nionization is 30% smaller than was previously measured in a single pass\nexperiment using an ion beam with an unknown metastable fraction. We also find\nsome significant differences between our experimental cross section for single\nionization and recent distorted wave (DW) calculations. The DW Maxwellian EISI\nrate coefficient for Fe^11+ forming Fe^12+ may be underestimated by as much as\n25% at temperatures for which Fe^11+ is abundant in collisional ionization\nequilibrium. This is likely due to the absence of 3s excitation-autoionization\n(EA) in the calculations. However, a precise measurement of the cross section\ndue to this EA channel was not possible because this process is not\ndistinguishable experimentally from electron impact excitation of an n=3\nelectron to levels of n > 44 followed by field ionization in the charge state\nanalyzer after the interaction region. Our experimental results also indicate\nthat the double ionization cross section is dominated by the indirect process\nin which direct single ionization of an inner shell 2l electron is followed by\nautoionization resulting in a net double ionization.\n