# Electron-Impact Ionization of P-like Ions forming Si-like Ions

Abstract: We have calculated electron-impact ionization (EII) for P-like systems from P to Zn15 + forming Si-like ions. The work was performed using the flexible atomic code (FAC) which is based on a distorted-wave approximation. All 3l → nl' (n = 3-35) excitation-autoionization (EA) channels near the 3p direct ionization threshold and 2l → nl' (n = 3-10) EA channels at the higher energies are included. Close attention has been paid to the detailed branching ratios. Our calculated total EII cross sections are compared both with previous FAC calculations, which omitted many of these EA channels, and with the available experimental results. Moreover, for Fe11 +, we find that part of the remaining discrepancies between our calculations and recent measurements can be accounted for by the inclusion of the resonant excitation double autoionization process. Lastly, at the temperatures where each ion is predicted to peak in abundances in collisional ionization equilibrium, the Maxwellian rate coefficients derived from our calculations differ by 50%-7% from the previous FAC rate coefficients, with the difference decreasing with increasing charge.

## Summary (1 min read)

### 1. INTRODUCTION

- 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).
- For this reason, the distorted wave (DW) method has been most widely employed to generate EII data for astrophysical plasma (Kallman & Palmeri 2007; Dere 2007).
- The development of ion storage rings combined with merged electron beams has helped to overcome this experimental limitation.
- In Section 3 the calculated total EII cross sections and rate coefficients are shown for the selected ions and compared with available experiments and with the previous theoretical works of Dere (2007).

### 2. THEORETICAL CALCULATIONS

- The authors have calculated EII cross sections based on the approach and ionization channels detailed in Kwon & Savin (2012).
- In addition to the above, the authors also calculated REDA for Fe11+.
- For the first order pertur- bation theory used in the conventional DW approach, the interaction potential is taken as a Coulomb field generated from an arbitrary effective charge, due to the screening of the nucleus by the continuum and bound electrons, and neglecting the long range interaction between the scattered and ejected continuum electrons (Macek & Botero 1992).
- As the authors show below, optimization on the 3s23p3 configuration yielded good agreement with the available experimental results for S+, Cl2+, and Ar3+ as well as for Fe11+.

### 3. RESULTS

- The calculated DI+EA+REDA cross section for ground state Fe11+ is shown in Figure 1.
- The authors new FAC calculations for S+ are in good agreement with the measurements by Yamada et al. (1988) and Djurić et al. (1993) near the ionization threshold and also at peak as shown in Figure 3.
- But at higher energies their calculations agree better with the results of Djurić et al. (1993) than with those of Yamada et al. (1988).
- The authors find that the peak DI cross sections for the metastable levels are larger by only 8% for S+ compared to the ground state.
- For S+ their calculated rate coefficient shows a +77% difference from the rate coefficient derived from the measurement by Yamada et al. (1988).

### 4. SUMMARY

- The authors have calculated EII for ground state P-like systems from P to Zn15+ forming Si-like ions.
- The calculations were performed using FAC within a DW approximation.
- The authors calculated total DI+EA cross sections are compared with the available experimental results for P, S+, Cl2+, Ar3+, and Ni+13+ and with the previous FAC calculations for Ar3+–Zn15+.
- Further theoretical and experimental work is required to resolve the discrepancy.

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