Symmetry-favoured double photoionization of argon

Experiments where the energies and angles of the two emitted electrons are resolved have been performed for double photoionization of argon in the valence shells. So far such experiments were always restricted to the ns(2)np(4) final states of doubly charged rare gases and to helium. Here we report on first measurements for the 3s3p(5) P-1(o) state of Ar2+ and we show that the P-1(o) symmetry of the residual ion leads to a situation which is completely new compared with all previous studies, since the two symmetries which are allowed for the pair of outgoing electrons (S-1(e) and D-1(e)) both have a 'favoured' character. As a consequence the observed angular patterns exhibit a sharp maximum when the two electrons fly in opposite directions with equal energies, whereas a node exists in the unfavoured cases such as helium. Dynamical effects which appear when the photon energy is varied are also discussed.