Dissociative recombination of HeH+ isotopes with an ultra-cold electron beam from a superconducting electron cooler in a storage ring

The dissociative recombinations of (HeH+)-He-3, (HeH+)-He-4, (HeD+)-He-3 and (HeD+)-He-4 were studied with an extremely low-temperature electron beam of the order of 1 meV produced by a superconducting electron cooler at the storage ring TARN II. The dissociative recombination spectra show a fine new structure and a clear isotope dependence in the low-energy region. Those features are well reproduced by the theoretical calculations based on a multichannel quantum-defect theory including rotational motion and discretized dissociative states. According to the theory, the newly observed structure is evidence for a new indirect mechanism where the intermediate states are not vibrationally excited Rydberg states but virtual dissociative states.