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

Relative yields of Ca+ and Ca- ions from thermal-energy collisions between highly excited Ca-* 4snd D-1 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 Ca- and a ground-state 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 Ca- is 24+/-1.4 meV.