Abstract

When two electrons are excited from the ground state of an atom, qualitatively new characteristics appear. This is particularly so with increasing excitation toward the ionization limit for the two-electron continuum. The increasing liberation of the electrons from the dominant central field in the atom causes greater relative importance of their mutual interaction. The independent particle picture becomes inadequate, especially for excitations in which the electrons remain comparable in their excitation. Such states are distinguished from those having disparate excitation in that, for the latter, independent particle labels for the radial motion (inner and outer electron) remain appropriate, but for the former a language that treats the two on par throughout is required. The two classes have been dubbed valley and ridge states. When viewed in joint coordinates of the pair of electrons, the potential surface of the system displays valleys in which one of the radial coordinates is small and a saddle region in which the two electrons remain comparable. Ridge states reside in the saddle, a long series having been observed for the first time in the He- system. Their description requires the use throughout of new quantum numbers characterizing the pair as a single entity. These matters and their experimental implications are discussed.

© 1985 Optical Society of America