Evidence for a stable negative ion of calcium

Abstract

We present experimental evidence for the existence of a stable ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ ion formed in the bound (4${\mathrm{s}}^{2}$4p${)\mathrm{}}^{2}$P state. This result represents the first report of a stable negative ion for a group-IIA element. The structure of ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ was determined by means of photoelectron detachment spectroscopy. The electron affinity of Ca was measured to be 0.43\ifmmode\pm\else\textpm\fi{}0.007 eV, which is in good agreement with a recent calculation.