Abstract
We have calculated electron-impact ionization (EII) for initially P-like systems for ions with an even proton number Z from S+ to Zn15+. We used the flexible atomic code (FAC) which is based on a distorted-wave (DW) approximation. In our work, 3l → nl (n = 4 − 35) excitation-autoionization (EA) channels near the 3p direct ionization threshold and 2l → nl' (n = 3 – 10) EA channels at the higher energies are included, along with the detailed branching ratios. Our calculated EII cross sections are compared both with previous FAC calculations, which omitted many of these EA channels, and with the available experiments.
Highlights
Modeling and interpreting spectra of collisionally ionized astrophysical plasmas requires accurate calculations for the underlying charge state distribution (CSD; Landi & Landini 1999; Kallman & Palmeri 2007; Bryans et al 2009)
Results with ion storage ring data, we must convolve the theoretical data with the flattened Maxwellian distribution of the experiment, which is described by parallel T and transverse T⊥ temperatures with respect to the electron beam direction (Schippers et al 2001)
In our previous electron-impact ionization (EII) calculation for ground state Fe11+ (Kwon & Savin 2012), the 2 → 3 EA channel appeared to turn on about 20 eV higher in energy than the experimental data. We hypothesized that this difference could be due to neither resonant excitation double autoionization (REDA) nor resonant excitation auto double ionization (READI) being accounted for in our previous calculations
Summary
Modeling and interpreting spectra of collisionally ionized astrophysical plasmas requires accurate calculations for the underlying charge state distribution (CSD; Landi & Landini 1999; Kallman & Palmeri 2007; Bryans et al 2009). The distorted wave (DW) method has been most widely employed to generate EII data for astrophysical plasma (Kallman & Palmeri 2007; Dere 2007) Testing of these theoretical methods has been performed for decades using benchmark experimental measurements. At temperatures where Fe11+ forms in collisional ionization equilibrium (CIE; Bryans et al 2009) the rate coefficient derived from our calculation lies within 11% of the experimentally derived rate coefficient and is in better agreement with the measurement of Hahn et al (2011a) than the previous FAC results of Dere (2007) which differed by up to 25% from the measurement.
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