WANNIER THEORY FOR DOUBLE PHOTOIONIZATION OF NOBLE-GASES

The Wannier theory not only gives the threshold laws for processes such as near-threshold double photoionization, but also predicts an angular correlation between the two escaping electrons. Within the frame of this theory and using an LS scheme we derive the triple, double differential and integral cross sections, for noble gases ions left in the ground or first excited states. Our approach should allow a direct comparison with forthcoming experiments, as it uses the individual angles of the two electrons. In the case of unfavoured final states, which have a node when the angle between these electrons is 180-degrees, the triple differential cross sections depend critically on the angular correlation. We give their detailed structure in space for double ionization by both polarized and unpolarized photons. We also show that a particular choice of Euler angles allows a straightforward integration which leads to the double differential cross sections. These are expressed in terms of the usual anisotropy parameters beta, for which explicit expressions are obtained. Finally by performing a further integration we show that an angular form factor appears in each integral cross section. The form factors for the unfavoured final states, although smaller, are not negligible compared with that of the 3P-degrees favoured state.