Recent experiments on near-threshold electron-impact excitation of multiply charged ions

Some recent measurements of excitation of multiply charged ions by electrons studied in beam-beam experiments are highlighted. The emphasis is on absolute total cross sections measured with the merged electron-ion beams energy-loss (MEIBEL) technique, al- though some results obtained with the crossed-beams fluorescence method are also presented. The MEIBEL technique allows the investigation of optically-allowed and forbidden transi- tions with sufficient energy resolution, typically about 0.2 eV, to resolve resonance structures in the cross sections. Results from the JILA/ORNL MEIBEL experiment on dipole-allowed transitions in several ions demonstrate the success of various theoretical methods in predicting cross sections in the absence of resonances. Comparisons of R-matrix calculations and mea- sured cross sections for spin-forbidden transitions in Mg-like Si and Ar , however, show that further refinements to the theory are needed in order to more accurately predict cross sec- tions involving significant contributions from dielectronic resonances and interactions between neighboring resonances.