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 ionization (EISI) of Fe 11+ forming Fe 12+ and electron impact double ionization (EIDI) of Fe 11+ forming Fe 13+ . The measurements cover the center-of-mass energy range from approximately 230 eV to 2300 eV. The experiment was performed using the heavy-ion storage ring TSR located at the Max-Planck-Institut für Kernphysik in Heidelberg, Germany. The storage ring approach allows nearly all metastable levels to relax to the ground state before data collection begins. We find that the cross section for single ionization is 30% smaller than was previously measured in a single-pass experiment using an ion beam with an unknown metastable fraction. We also find some significant differences between our experimental cross section for single ionization and recent distorted wave (DW) calculations. The DW Maxwellian EISI rate coefficient for Fe 11+ forming Fe 12+ may be underestimated by as much as 25% at temperatures for which Fe 11+ is abundant in collisional ionization equilibrium. This is likely due to the absence of 3s excitation-autoionization (EA) in the calculations. However, a precise measurement of the cross section due to this EA channel was not possible because this process is not distinguishable experimentally from electron impact excitation of an n = 3 electron to levels of n 44 followed by field ionization in the charge state analyzer after the interaction region. Our experimental results also indicate that the EIDI cross section is dominated by the indirect process in which direct single ionization of an inner shell 2l electron is followed by autoionization, resulting in a net double ionization.

Storage ring cross section measurements for electron impact ionization of Fe8+

“…The procedures used here are basically the same as those we have described for previous EII measurements and are described in detail in Linkemann et al (1995), Hahn et al (2010Hahn et al ( , 2011aHahn et al ( ,b, 2012aHahn et al ( ,b, 2013, and Bernhardt et al (2014). Additionally, recombination measurements for Fe 7+ were performed at TSR by Schmidt et al (2008), using similar methods.…”

“…The EII cross section was calculated by subtracting the background rate from the measured ionization count rate and normalizing by the stored ion number and the electron density (e.g., Hahn et al 2011a). The uncertainties in the measurement are summarized in Table 1.…”

“…We have derived plasma ionization rate coeffieicnts α I (T e ) from the measured cross sections, as described in Hahn et al (2011a). Figure 4 shows the inferred plasma rate coefficient.…”

Influence of Electron-Impact Multiple Ionization on Equilibrium and Dynamic Charge State Distributions: A Case Study Using Iron

“…Double-ionization cross sections for Fe 1+ and Fe 3+ -Fe 6+ were measured by Stenke et al (1999). Fits to these experimental data Hahn et al (2011a). The solid curve is the cross section used here, which is described in Table 1.…”

Storage ring at HIE-ISOLDE

“…High T e DR is believed to be well understood theoretically, in part due to an extensive series of DR measurements performed by us at TSR [168,169]. EII is less well understood, as our work at TSR has shown [170,171]. Theoretical calculations are largely untested against reliable experimetal work.…”