SHAPE RESONANCE IN CA- PHOTODETACHMENT AND THE ELECTRON-AFFINITY OF CA((1)S)

It was only recently found that a stable negative ion Ca - 2P exists below the Ca(1S) ground state. An electron affinity of 43 +/- 7 meV was obtained experimentally, and numerous theoretical calculations have now found values between 0 and 100 meV. We report the first photodetachment threshold measurements on Ca -, which display a p-wave threshold above Ca 4s4p 3P and a large p-wave shape resonance above Ca 4s4p 1P. Both threshold energies obtained from the data agree with a new electron affinity of 18.4 +/- 2.5 meV for Ca(1S).