Electron-ion recombination of Mg6 + forming Mg5 + and of Mg7 + forming Mg6 +: laboratory measurements and theoretical calculations

M. Lestinsky, Nigel Badnell, D. Bernhardt, D. Bing, M. Grieser, M. Hahn, J. Hoffmann, B. Jordon-Thaden, C. Krantz, O. Novotný, D.A. Orlov, R. Repnow, A. Shornikov, A. Müller, S Schippers, A. Wolf, D. W. Savin

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Abstract

We have measured electron–ion recombination for C-like Mg6+ forming Mg5+, and for B-like Mg7+ forming Mg6+. These studies were performed using a merged electron–ion beam arrangement at the TSR heavy ion storage ring located in Heidelberg, Germany. Both primary ions have metastable levels with significant lifetimes. Using a simple cascade model we estimate the population fractions in these metastable levels. For the Mg6+ results, we find that the majority of the stored ions are in a metastable level, while for Mg7+ the metastable fraction is insignificant. We present the Mg6+ merged beams recombination rate coefficient for DR via N = 2 → N = 2 core electron excitations (ΔN = 0 DR) and for Mg7+ via 2 → 2 and 2 → 3 core excitations. Taking the estimated metastable populations into account, we compare our results to state-of-the-art multiconfiguration Breit–Pauli theoretical calculations. Significant differences are found at low energies where theory is known to be unreliable. Moreover, for both ions we observe a discrepancy between experiment and theory for ΔN = 0 DR involving capture into high-n Rydberg levels and where the stabilization is primarily due to a radiative transition of the excited core electron. This is consistent with previous DR experiments on M-shell iron ions which were performed at TSR. The large metastable content of the Mg6+ ion beam precludes generating a plasma recombination rate coefficient (PRRC). However, this is not an issue for Mg7+ and we present an experimentally derived Mg7+ PRRC for plasma temperatures from 400 K to 107 K with an estimated uncertainty of less than 27% at a 90% confidence level. We also provide a fit to our experimentally derived PRRC for use in plasma modeling codes.
Original languageEnglish
Article number40
Number of pages13
JournalAstrophysical Journal
Volume40
Issue number1
DOIs
Publication statusPublished - 24 Sep 2012

Keywords

  • atomic data
  • atomic processes
  • electron-ion recombination
  • plasma recombination rate coefficient
  • plasma temperature
  • plasma modelling codes
  • cosmic atomic plasma
  • ionization balance calculations
  • dielectronic recombination

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