Enhancement of electron–ion recombination rates at low energy range in the heavy ion storage ring CSRm*

Abstract

Recombination of Ar14+, Ar15+, Ca16+, and Ni19+ ions with electrons has been investigated at low energy range based on the merged-beam method at the main cooler storage ring CSRm in the Institute of Modern Physics, Lanzhou, China. For each ion, the absolute recombination rate coefficients have been measured with electron–ion collision energies from 0 meV to 1000 meV which include the radiative recombination (RR) and also dielectronic recombination (DR) processes. In order to interpret the measured results, RR cross sections were obtained from a modified version of the semi-classical Bethe and Salpeter formula for hydrogenic ions. DR cross sections were calculated by a relativistic configuration interaction method using the flexible atomic code (FAC) and AUTOSTRUCTURE code in this energy range. The calculated RR + DR rate coefficients show a good agreement with the measured value at the collision energy above 100 meV. However, large discrepancies have been found at low energy range especially below 10 meV, and the experimental results show a strong enhancement relative to the theoretical RR rate coefficients. For the electron–ion collision energy below 1 meV, it was found that the experimentally observed recombination rates are higher than the theoretically predicted and fitted rates by a factor of 1.5 to 3.9. The strong dependence of RR rate coefficient enhancement on the charge state of the ions has been found with the scaling rule of q 3.0, reproducing the low-energy recombination enhancement effects found in other previous experiments.

Absolute rate coefficients for dielectronic recombination of Na-like Kr25+

“…However, the deviations from the hydrogenic RR model have a different behavior than that shown in Ref. [48]. While the origin of this discrepancy remains unclear, there is no indication that the measured smooth rate should be not attributed to DR.…”

“…The dashed olive and dashed red curves are the FAC calculation in which the field ionization effect was considered with n cut-off = 150 and 300, respectively. The shot-dotted blue curve is the AUTOSTRUCTURE calculated DR spectrum with n max = 1000. peak appears around 1.5 eV for Kr 25+ , which is already far off the RR rate coefficient enhancement region, the RR rate enhancement will have minor impact on our results and the contribution from RR and background can be fully removed with optimized data analysis [48,51], as shown in Fig. 2.…”

Experimental dielectronic recombination rate coefficients for lithium-like 40Ca17+

“…The DR experiment of Li-like 40 Ca 17+ ions was carried out using accelerator facility at Institute of Modern Physics (IMP), Lanzhou. The same setup and technique used for our other recombination experiments at CSRm can be found with detailed description in references [51,76,77]. In the present paper, we will briefly describe the most relevant experimental features related to the lithium-like 40 Ca 17+ ions recombination with electrons.…”