The Wannier theory not only gives the threshold laws far 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", the triple differential cross sections depend critically on the angular correlation. We give their detailed structure in space for doubleionization by bath polarized andunpolarized 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 p, 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 unfavaured final states, although smaller, are not negligible compared with that of the ' P favoured state.

We observe the formation in a single-photon transition of two core holes, each at a different carbon atom of the C2H2 molecule. At a photon energy of 770.5 eV, the probability of this 2-site core double ionization amounts to 1.6 ± 0.4% of the 1-site core double ionization. A simple theoretical model based on the knockout mechanism gives reasonable agreement with experiment. Spectroscopy and Auger decays of the associated double core hole states are also investigated.

The formation of hollow molecules (with a completely empty K shell in one constituent atom) through single-photon core double ionization has been demonstrated using a sensitive magnetic bottle experimental technique combined with synchrotron radiation. Detailed properties are presented such as the spectroscopy, formation, and decay dynamics of the N(2)(2+) K(-2) main and satellite states and the strong chemical shifts of double K holes on an oxygen atom in CO, CO2, and O2 molecules.