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

Lestinsky, M. and Badnell, Nigel and Bernhardt, D. and Bing, D. and Grieser, M. and Hahn, M. and Hoffmann, J. and Jordon-Thaden, B. and Krantz, C. and Novotný, O. and Orlov, D.A. and Repnow, R. and Shornikov, A. and Müller, A. and Schippers, S and Wolf, A. and Savin, D. W. (2012) Electron-ion recombination of Mg6 + forming Mg5 + and of Mg7 + forming Mg6 + : laboratory measurements and theoretical calculations. Astrophysical Journal, 40 (1). ISSN 0004-637X

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    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.