Ca- and its elusive properties

This 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 finding energy for the ground state Ca-2P of only 18.4±2.5 meV compared with the previous value of 43± meV, and have also confirmed earlier observations of the metastable 4P state. Some important uncertainties still exist and are noted. 32 refs., 1 tab., 9 figs