Experimental dielectronic recombination rate coefficients for Na-like S VI and Na-like Ar VIII

¹ Department of Atomic Physics, Stockholm University, 10691 Stockholm, Sweden e-mail: istvan.orban@physto.se
² Department of Physics, Marmara University, 81040 Istanbul, Turkey
³ Manne Siegbahn Laboratory, Stockholm University, 11814 Stockholm, Sweden
⁴ Gesellschaft für Schwerionenforschung, 64291 Darmstadt, Germany
⁵ Institut für Kernphysik, Universität Frankfurt, 60486 Frankfurt, Germany

Received: 5 February 2009
Accepted: 22 February 2009

Abstract

Aims. Absolute recombination rate coefficients for two astrophysically relevant Na-like ions are presented.

Methods. Recombination rate coefficients of and are determined from merged-beam type experiments at the CRYRING electron cooler. Calculated rate coefficients are used to account for recombination into states that are field-ionized and therefore not detected in the experiment.

Results. Dielectronic recombination rate coefficients were obtained over an energy range covering core excitations. For Na-like Ar a measurement was also performed over the type of resonances. In the low-energy part of the  spectrum, enhancements of more than one order of magnitude are observed as compared to the calculated radiative recombination. The plasma recombination rate coefficients of the two Na-like ions are compared with calculated results from the literature. In the  K range, large discrepancies are observed between calculated plasma rate coefficients and our data. At higher temperatures, above 10⁵ K, in the case of both ions our data is 30% higher than two calculated plasma rate coefficients, other data from the literature having even lower values.

Conclusions. Discrepancies below 10⁴ K show that at such temperatures even state-of-the-art calculations yield plasma rate coefficients that have large uncertainties. The main reason for these uncertainties are the contributions from low-energy resonances, which are difficult to calculate accurately.