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 ${\mathrm{C}}^{4+}$ and ${\mathrm{O}}^{6+}$ ions and for Li-like ${\mathrm{C}}^{3+}$ and ${\mathrm{O}}^{5+}$ 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 ${\mathrm{C}}^{3+}$ 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 ${\mathrm{C}}^{3+}$ and around 7 V/cm for ${\mathrm{O}}^{5+}$. 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 ${10}^{\mathrm{-}15}$ to ${10}^{\mathrm{-}14}$ ${\mathrm{cm}}^2$ are found.