Dielectronic recombination from the ground state of heliumlike carbon ions

The cross section for dielectronic recombination of ${\mathrm{C}}^{4+}$ ions in the 1${\mathit{s}}^2$ ground state has been measured with an energy resolution of ≊2 eV at the heavy-ion storage ring TSR (at Heidelberg) up to the 1s2s${(}^{1,3}$S) and 1s2p${(}^{1,3}$P) excitation thresholds just below 310 eV. Resonance energies and absolute cross sections are found to be well reproduced by theoretical calculations except in a region extending ≊5 eV below the almost degenerate excitation thresholds 1s2s${(}^1$S) and 1s2p${(}^3$P). These discrepancies imply that theory overestimates the cross section, energy-integrated over all dielectronic resonances, by ≊10%.