Multiphoton Excitation of Doubly Excited States of Two-Electron Atoms
One of the outstanding problems in atomic physics is to better understand the dynamics of highly excited two-electron systems. In 1977, I.C. PERCIVAL  coined the term “planetary atoms” to describe these atoms in which both of the electrons are so highly excited that their mutual perturbations are large enough to introduce dramatic angular correlations in their motion. He introduced a semiclassical method-quantizing classical two-electron orbits-to predict some scaling laws. Others have attempted to solve this problem from a purely quntum mechanical approach. HERRICK and SINANOGLU  have employed group theory methods to classify energy levels for the He 3≬ 3≬’ Their approach was strongly suggestive that these atoms vibrated and rotated with the electrons localized on opposite sides of the nucleus-more like a “molecular atom”. FANO and others  have analyzed the problem using hyperspherical coordinates and found that, at least for the case of near threshold ionization, both electrons are highly correlated in angle as they move along a “Wannier ridge”, a local maximum in the potential energy surface.
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