Ca? and Its Elusive Properties

The historical progression of theoretical and experimental information on the properties of the two electronic states of the Ca- ion is reviewed. New results are described that have revealed a shape resonance and a Cooper minimum, and have yielded a much lower electron binding energy for the ground state Ca- 2p of only 18·4±2·5 meV compared with the previous value of 43±7 meV, and have also confirmed earlier observations of the metastable 4p state. Some important uncertainties still exist and are noted.