Shape resonance inCa−photodetachment and the electron affinity of Ca(1S)

It was only recently found that a stable negative ion ${\mathrm{Ca}}^{\mathrm{-}}$ ${}_{}{}^2{}_{}{}^{}$P exists below the Ca${(}^1$S) 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 ${\mathrm{Ca}}^{\mathrm{-}}$, which display a p-wave threshold above Ca 4s4p ${}_{}{}^3{}_{}{}^{}$P and a large p-wave shape resonance above Ca 4s4p ${}_{}{}^1{}_{}{}^{}$P. Both threshold energies obtained from the data agree with a new electron affinity of 18.4±2.5 meV for Ca${(}^1$S).