High resolution low-energy dielectronic recombination rate coefficients of beryllium-like germanium: QED test bench for two-valence-electron systems

Low-energy dielectronic recombination resonances of beryllium-like Ge28+ have been studied by electron collision spectroscopy in the heavy-ion storage ring TSR using a twin-electron beam technique combined with an ultracold photoelectron source. Rich resonance structure below 1.7 eV is observed which mainly arises from resonant capture of the free electron to 91 Rydberg states accompanied by excitation of inner valence electrons from the 2s2(1S0) ground state to 2s2p(1P1). The positions of resonances below 0.3 eV are measured with an accuracy better than 0.5 meV. Accurate calculations of Rydberg binding energies for these resonances are still required in order to derive a precise value for the 1S0 - 1P1 transition energy, which may give the opportunity for high-accuracy testing of QED corrections to atomic energy levels for a two valence electron system.