Recombination Measurements of the Heaviest Ions

Dielectronic recombination and radiative recombination have been studied for several lithium-like ions (Au⁷⁶⁺, Pb⁷⁹⁺, Bi⁸⁰⁺ and U⁸⁹⁺) as well as for hydrogen-like Bi⁸²⁺ and completely stripped Bi⁸³⁺. Experiments were performed at the electron cooler of the storage ring ESR of the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt. In the dielectronic recombination spectra analyzed so far for the lithium-like ions, 2s_1/2 → 2p_1/2 excitations with capture of the free electron into Rydberg states with n ≥ 20 and 2s_1/2 → 2p_3/2 excitations with capture into n = 5 (uranium) and n = 6 (gold, lead, bismuth) are observed. These measurements provide a stringent test of atomic structure and recombination theory for high-Z heavy-ions. In particular, the extrapolation of the 2p_1/2nl_j sequence (n ≥ 20) to the series limit renders possible to extract the 2s_1/2 → 2p_1/2 Lamb-shift in lithium-like high-Z ions at an expected accuracy level which can test QED theory.