Electron-ion recombination rate coefficients of Be-like 40Ca16+

Wang, S. X. and Xu, X. and Huang, Z. K. and Wen, W. Q. and Wang, H. B. and Khan, N. and Preval, S. P. and Badnell, N. R. and Schippers, S. and Mahmood, S. and Dou, L. J. and Chuai, X. Y. and Zhao, D. M. and Zhu, X. L. and Mao, L. J. and Ma, X. M. and Li, J. and Mao, R. S. and Yuan, Y. J. and Tang, M. T. and Yin, D. Y. and Yang, J. C. and Ma, X. and Zhu, L. F. (2018) Electron-ion recombination rate coefficients of Be-like 40Ca16+. Astrophysical Journal, 862 (2). ISSN 0004-637X

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    Electron–ion recombination rate coefficients for beryllium-like calcium ions in the center of mass energy from 0 to51.88 eV have been measured by means of the electron–ion merged-beam technique at the main cooler storage ringat the Institute of Modern Physics in Lanzhou, China. The measurement energy range covers the dielectronicrecombination (DR) resonances associated with the 2s2 1S0  -> 2s2p 3P0,1,2, 1P1 core excitations and the trielectronicrecombination (TR) resonances associated with the 2s2 1S0  -> 2p2 3P0,1,2, 1D2, 1S0 core excitations. In addition,the AUTOSTRUCTURE code was used to calculate the recombination rate coefficients for comparison with theexperimental results. Resonant recombination originating from parent ions in the long-lived metastable state 2s2p 3P0 ions has been identified in the recombination spectrum below 1.25 eV. A good agreement is achievedbetween the experimental recombination spectrum and the result of the AUTOSTRUCTURE calculations whenfractions of 95% ground-state ions and 5% metastable ions are assumed in the calculation. It is found thatthe calculated TR resonance positions agree with the experimental peaks, while the resonance strengths areunderestimated by the theoretical calculation. Temperature dependent plasma rate coefficients for DR and TR inthe temperature range of 103–108 K were derived from the measured electron–ion recombination rate coefficientsand compared with the available theoretical results from the literature. In the temperature range of photoionizedplasmas, the presently calculated rate coefficients and the recent results of Gu & Colgan et al. are up to 30% lowerthan the experimentally derived ones, and the older atomic data are even up to 50% lower than the presentexperimental result. This is because strong resonances situated below electron–ion collision energies of 50 meVwere underestimated by the theoretical calculation, which also has a severe influence on the rate coefficients inlow-temperature plasmas. In the temperature range of collisionally ionized plasmas, agreement within 25% wasfound between the experimental result and the present calculation as well as the calculation by Colgan et al. Thepresent result constitutes a set of benchmark data for use in astrophysical modeling.