Electron-ion recombination rate coefficients for beryllium-like calcium ions in the cen-

16 Electron-ion recombination rate coefficients for beryllium-like calcium ions in the cen17 ter of mass energy from 0 to 51.88 eV have been measured by employing the electron18 ion merged-beam technique at the cooler storage ring CSRm at the Institute of Mod19 ern Physics, Lanzhou, China. The measurement energy range covers the dielectronic 20 recombination (DR) resonances associated with the 2s S0 → 2s2p P0,1,2, P1 core 21 Corresponding author: W. Q. Wen wenweiqiang@impcas.ac.cn Corresponding author: X. Ma x.ma@impcas.ac.cn Corresponding author: L. F. Zhu lfzhu@ustc.edu.cn 2 Shu-Xing Wang et al. excitations and the trielectronic recombination (TR) resonances associated with the 22 2s S0 → 2p P0,1,2,D2, S0 core excitations. In addition, theoretical calculations 23 of the recombination rate coefficients have been performed for comparison with the 24 experimental results using the state-of-the-art multi-configuration Breit-Pauli atomic 25 structure code. Resonant recombination originating from parent ions in the long-lived 26 metastable state 2s2p P0 ions has been identified in the recombination spectrum below 27 1.25 eV. A good agreement is achieved between the experimental recombination spec28 trum and the result of the AUTOSTRUCTURE calculations when fractions of 95% 29 ground-state ions and 5% metastable ions are assumed in the calculation. It is found 30 that the calculated TR resonance positions agree with the experimental peaks while 31 the resonance strengths are much underestimated by the theoretical calculation. Tem32 perature dependent plasma rate coefficients for DR and TR in the temperature range 33 10−10 K were derived from the measured electron-ion recombination rate coefficients 34 and compared with the available theoretical results from the literature. In the temper35 ature range of photoionized plasmas, the presently calculated rate coefficients and the 36 recent results of Gu (2003) and Colgan et al. (2003) are up to 30% lower than the 37 experimentally derived plasma rate coefficients, and the older atomic data are even up 38 to 50% lower than the present experimental result. This is because strong resonances 39 situated below electron-ion collision energies of 50 meV were underestimated by the 40 theoretical calculation, which also has a severe influence on the rate coefficients in low 41 temperature plasmas. In the temperature range of collisionally ionized plasmas, agree42 ment within 25% was found between the experimental result and the present calculation 43 as well as the calculation by Colgan et al. (2003). The present result constitutes a set 44 of benchmark data for use in astrophysical modeling. 45