Double photoionization is a fundamental process in which one absorbed photon kicks out two electrons simultaneously from an atom. This is only possible through many-electron correlation which means the electrons are tightly packed in the atom like billiard balls in the diagram on the right. Only then is it possible to eject two balls in one hit, or two electrons by absorbing just a single photon.
Many-electron correlation plays important role in atomic physics and in physics of many-electron systems in general. In solids, because of larger electron densities, the many-electron correlation is responsible for such fundamental and important effects as ferromagnetism and superconductivity. Double photoionization is an ideal tool to study many-electron correlation. This process would be simply impossible if the electrons were independent particles.
The simplest target to study double photoionization is a system of two interacting electrons, i.e. the helium atom. There exist very accurate ground state wave functions for helium which account for the target many-electron correlation prior to absorption of the photon. However, unlike the billiard balls, the electrons remain correlated via the Coulomb force even after they left the atom.
Account of this type of correlation requires an elaborate technique which is called the convergent close-coupling method. With the account for the ground and final ionized state correlations, the double photoionization from the helium atom can be calculated very accurately, in many cases, more accurately than the experiment. The work is now underway to expand the existing theory to more complex atoms such as alkaline-earth and noble-gas atoms.
A. S. Kheifets, D. V. Fursa and Igor Bray
Two-electron photoionization of ground-state lithium
Phys. Rev. A 80, 063413, 2009
A. S. Kheifets and Igor Bray
Valence shell double photoionization of alkaline-earth atoms
Phys. Rev. A 75, 042703, 2007