Electron-impact double ionization of Ca ions

The absolute cross section for the double ionization of from threshold to 500 eV has been measured for the first time. A combination of electron beam modulation and fast energy-scanning techniques was used. At energies around 340 eV the cross section displays several steps and resonant peaks, which can be identified as arising from autoionization processes involving the excitation of a 2p electron. There is little evidence of structure elsewhere in the cross section, although some points of inflexion are apparent in the region below 90 eV.