Evidence for resonant formation ofCa−in collisions of Rydberg and ground-state Ca atoms

Relative yields of ${\mathrm{Ca}}^{+}$ and ${\mathrm{Ca}}^{\mathrm{-}}$ ions from thermal-energy collisions between highly excited ${\mathrm{Ca}}^{\mathrm{*}}$ 4snd ${}_{}{}^1{}_{}{}^{}$D Rydberg atoms and Ca ground-state atoms are measured as a function of n. The collisions take place within an effusive Ca atomic beam. The cross section for a collision resulting in a bound ${\mathrm{Ca}}^{\mathrm{-}}$ and a ground-state ${\mathrm{Ca}}^{+}$ is expected to be large when the binding energy of the excited Rydberg state nearly matches the electron affinity of Ca. We observe a large increase in positive and negative ions for n=25 only. This result strongly suggests that the binding energy of ${\mathrm{Ca}}^{\mathrm{-}}$ is 24±1.4 meV.