Accurate determination of dielectronic recombination resonances with lithiumlike argon

A highly accurate measurement of Δn=0 dielectronic recombination (DR) resonances with Ar¹⁵⁺ has been carried out at the ion storage ring CRYRING. The absolute energies of the DR resonances were determined by a method that uses the Schottky frequency and the knowledge of the length of the beam orbit in the ring. The accuracy in the absolute energy achieved in this experiment was ∼1%. Positions and widths of doubly excited states of Ar¹⁴⁺, as well as the DR cross sections, were calculated with relativistic many-body perturbation theory in an all-order formulation. Radiative corrections and mass polarization were needed to obtain good agreement with the experimental results.