RESONANT FORMATION OF CA-ASTERISK(4S25D(1)D(2)) LEVEL EXPLAINED BY AUTOIONIZATION THROUGH THE 3D(3( FROM THE CA)2)9F(5/2),J(PI)=2(-) RESONANCE/

The formation of Ca+ ions is measured in an effusive Ca beam by exciti ng through either the 4s4p(1)P(1) or the 4s5p(1)P(1) state to highly e xcited Ca(4snd(1)D(2)) Rydberg states. In agreement with previous wor k by McLaughlin and Duquette [Phys. Rev. Lett. 72, 1176 (1994)], when the Rydberg states are accessed through the 4s5p(1)P(1) state, we dete ct a large increase in Ca+ ions for n = 25; compared to other high-n R ydberg states. It had been reported (Fabrikant [Phys. Rev. A 48, R3411 (1993)]; McLaughlin and Duquette) that ''ground-state-Rydberg-state'' collisions cause this large increase in ion production and that the r esult provides a precise value of the electron affinity of Ca through the binding energy of the Rydberg electron. However, a variation of th e original experiment in which the same Rydberg states are accessed th rough the 4s4p(1)P(1) level instead of the 4s5p(1)P(1) level shows no enhancement of ions for any particular Rydberg level. With a three-las er experiment we show that the enhanced ionization effect for the 25d state is due to the 3d(3/2)9f(5/2), J(pi)=2(-) autoionizing resonance, which is coincidentally accessed from the Ca(4s25d(1)D(2)) state by t he 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 R-matrix calculations.