Measurement of doubly excited levels in lithiumlike and berylliumlike cobalt

We have measured the dielectronic satellite spectrum for heliumlike Co xxvi trapped and excited in an electron-beam ion trap using a high-resolution x-ray spectrometer in the von H\'amos geometry. By sweeping the electron-beam energy across the individual dielectronic recombination resonances, we have determined the relative resonance strengths of the strongest resonances populating doubly excited levels in lithiumlike and berylliumlike cobalt. Most measured dielectronic satellite strengths compare well with theory. A discrepancy with theoretical values is noted for the resonance strength associated with levels (1s1/22p3/2²{)}3/2$ and (1${s}1/2$2${s}1/2$2${p}1/2)1/2. Their values of 49.8±{}3.2×{}10^-20 cm² eV and 22.9±{}6.4×{}10^-20 cm² eV, respectively, are almost twice the size predicted by theory.