State-selective dielectronic-recombination measurements for He- and Li-like carbon and oxygen ions

Dielectronic recombination due to the 2s{→}2p excitation has been measured for metastable He-like C⁴⁺ and O⁶⁺ ions and for Li-like C³⁺ and O⁵⁺ ions. The measurements have been performed with a merged-beams technique. An energy resolution of 0.135 eV was obtained. The data for the Li-like ions are compared with distorted-wave calculations. For the low-lying resonances, good agreement is found between experiment and theory except for one resonance (2p4l) for C³⁺ that is very close to threshold. The contribution due to higher Rydberg states is in agreement with theory for external fields of 3 V/cm for C³⁺ and around 7 V/cm for O⁵⁺. The field present in the experiment is expected to be around 2 V/cm. Large and unexpected structures are observed in the He-like data. These have tentatively been explained in terms of interaction between different core configurations. For the metastable He-like ions extremely large cross sections of the order of 10^-15 to 10^-14 cm² are found.