In photodouble ionisation light is absorbed by an atom or molecule resulting in the emission of two electrons and the formation of a doubly-charged ion, i.e.:

The probability of ejecting two electrons with a single photon of light is typically 20-100 times less than ejecting one (fast) electron, thus making the experiment much harder to perform. Coincidence techniques are employed which involve correlating in time the energy-selected electrons to ensure the two detected electrons are from the same atom. Furthermore, novel electron energy/angle analysers have been specifically developed for this experiment which allow simultaneous measurement of electron pairs over a wide range of emission angles, so improving the overall detection efficiency without loss of resolution. This is particularly important as the experiments are performed at synchrotron radiation sources - major facilities for producing x-ray and ultra-violet radiation, and beam time is both expensive and limited.

The main motivation for this research is to determine the directions of the two ejected electrons, with respect to the polarised light, for a variety of possible electron energy conditions. Simple models, ones which are used to account for all the main features in atomic and molecular structure, assume the motion of the electrons to be independent from each other - which also implies that the double ionisation process cannot occur at all. Consequently, these studies extend our fundamental understanding of atomic and molecular structure and dynamics by providing tests for more complex models which focus on the correlated aspects of electron motion. For these reasons, we have concentrated our efforts on the simplest two-electron systems; helium and molecular hydrogen.