EVIDENCE FOR RESONANT FORMATION OF CA- IN COLLISIONS OF RYDBERG AND GROUND-STATE CA ATOMS

Relative yields of Ca[sup +] and Ca[sup [minus]] ions from thermal-energy collisions between highly excited Ca[sup *] 4[ital snd] [sup 1][ital D] Rydberg atoms and Ca ground-state atoms are measured as a function of [ital n]. The collisions take place within an effusive Ca atomic beam. The cross section for a collision resulting in a bound Ca[sup [minus]] and a ground-state Ca[sup +] 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 [ital n]=25 only. This result strongly suggests that the binding energy of Ca[sup [minus]] is 24[plus minus]1.4 meV.