Resonant formation of Ca+ from the Ca*(4s25d1D2) level explained by autoionization through the 3d{3/2}9f{5/2}, Jπ=2- resonance

The formation of Ca{sup +} ions is measured in an effusive Ca beam by exciting through either the 4{ital s}4{ital p}{sup 1}{ital P}{sub 1} or the 4{ital s}5{ital p}{sup 1}{ital P}{sub 1} state to highly excited Ca{asterisk}(4{ital snd}{sup 1}{ital D}{sub 2}) Rydberg states. In agreement with previous work by McLaughlin and Duquette [Phys. Rev. Lett. {bold 72}, 1176 (1994)], when the Rydberg states are accessed through the 4{ital s}5{ital p}{sup 1}{ital P}{sub 1} state, we detect a large increase in Ca{sup +} ions for {ital n}=25, compared to other high-{ital n} Rydberg states. It had been reported {bold (}Fabrikant [Phys. Rev. A {bold 48}, R3411 (1993)]; McLaughlin and Duquette{bold )} that {open_quote}{open_quote}ground-state{endash}Rydberg-state{close_quote}{close_quote} collisions cause this large increase in ion production and that the result provides a precise value of the electron affinity of Ca through the binding energy of the Rydberg electron. However, a variation of the original experiment in which the same Rydberg states are accessed through the 4{ital s}4{ital p}{sup 1}{ital P}{sub 1} level instead of the 4{ital s}5{ital p}{sup 1}{ital P}{sub 1} level shows no enhancement of ions for any particular Rydberg level. With a three-laser experiment we show that the enhanced ionization effect for the 25{italmore » d} state is due to the 3{ital d}{sub 3/2}9{ital f}{sub 5/2}, {ital J}{sup {pi}}=2{sup {minus}} autoionizing resonance, which is coincidentally accessed from the Ca(4{ital s}25{ital d}{sup 1}{ital D}{sub 2}) state by the 808-nm photon involved in the original experiment by McLaughlin and Duquette. The existence of this resonance close to the experimentally observed energy is also established by {ital R}-matrix calculations. {copyright} {ital 1996 The American Physical Society.}« less