Resonant formation of Ca+ from the Ca*(4s25 d1 D2 ) level explained by autoionization through the 3 d3/2 9 f5/2 , Jπ = 2− resonance

Abstract

The formation of ${\mathrm{Ca}}^{+}$ ions is measured in an effusive Ca beam by exciting through either the 4s4${\mathit{p}}^{1}$${\mathit{P}}_{1}$ or the 4s5${\mathit{p}}^{1}$${\mathit{P}}_{1}$ state to highly excited Ca*(4${\mathit{snd}}^{1}$${\mathit{D}}_{2}$) Rydberg states. In agreement with previous work by McLaughlin and Duquette [Phys. Rev. Lett. 72, 1176 (1994)], when the Rydberg states are accessed through the 4s5${\mathit{p}}^{1}$${\mathit{P}}_{1}$ state, we detect a large increase in ${\mathrm{Ca}}^{+}$ ions for n=25, compared to other high-n Rydberg 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 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 4s4${\mathit{p}}^{1}$${\mathit{P}}_{1}$ level instead of the 4s5${\mathit{p}}^{1}$${\mathit{P}}_{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 25d state is due to the 3${\mathit{d}}_{3/2}$9${\mathit{f}}_{5/2}$, ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${2}^{\mathrm{\ensuremath{-}}}$ autoionizing resonance, which is coincidentally accessed from the Ca(4s25${\mathit{d}}^{1}$${\mathit{D}}_{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 R-matrix calculations. \textcopyright{} 1996 The American Physical Society.