Correlations of two excited electrons

Advances over the last twenty years are reported, with the emphasis on describing correlations through the distribution of an electron pair's wavefunction in its configuration space. The symmetry of the Schrodinger equation and its eigenfunctions are thus viewed and found to be important. The radial correlations are more significant for the mechanics of the pair than the angular correlations. Connections with other approaches to the subject are described. Earlier studies concerning doubly excited helium and the hydrogen negative ion are related to Wannier's threshold law (1953) for ionisation by electron collision, but these two subjects are not yet fully connected. Recent studies on the alkaline earths and helium negative ion are described and viewed as a prelude to a renewed analysis and interpretation of the optical spectra and electron collisions for atoms with partially filled valence shells, which afford ample opportunity for the manifestation of strong correlations.