Fine structure of negative ions of alkaline-earth-metal atoms

Fine-structure intervals np1/2-np3/2 (np1/2 is the ground state) are calculated for negative ions using relativistic many-body perturbation theory: Ca^- 4p, 56 cm^-1; Sr^- 5p, 178 cm^-1; Ba^- 6p, about 460 cm^-1; and Ra^- 7p, 1341 cm^-1. Comparison of the fine-structure interval with affinity: Ca^- 10%, Sr^- 22%, Ba^- 30%; for Ra^- the fine-structure interval is larger than the affinity. Thus the Ra^- 7p3/2 state is a resonance in the continuous spectrum with energy 0.018 eV. This means that the relativistic spin-orbit interaction plays a determining role in slow electron-radium scattering.