Abstract

The atomic three-body Coulomb problem of two electrons moving in the field of a positive ion remains a challenge in atomic physics. In 1963 Madden and Codling found the first experimental evidence for two-electron correlations in a single-photon absorption experiment on doubly-excited helium [1], using synchrotron radiation in the 19-21 mn wavelength region. More evidence for electron correlation was obtained in emission spectroscopic and electron scattering experiments on helium. Common in these experiments is their limited resolution. Due to the large excitation energies involved, doubly-excited states of helium are inaccessible for narrow-band laser systems. Laser spectroscopy can be performed on other two-electron systems such as alkaline-earth atoms. The heavier alkaline-earth atoms (Ca, Sr, Ba) have two loosely bound valence electrons and are therefore easily excited by visible or UV laser radiation. They differ from helium due to their spatially extended 2⁺-core. When an electron penetrates the residual core its binding energy increases. As core penetration strongly depends on the orbital angular momentum ℓ of the valence electron, it removes the ℓ-degeneracy which is characteristic for helium. For high - ℓ the electron’s wavefuction has no overlap with the core due to the centrifugal barrier effect. By promoting both valence electrons to high- ℓ orbitals a true three-body Coulomb state may be excited.

© 1996 IEEE