Signature of the target two-electron momentum space wavefunction in the (e, 3e) angular distributions from the double ionization of helium and argon

New coplanar (e, 3e) experiments for double ionization (DI) of helium (1s 2 ) and argon (3p 2 ) at about 600 eV incident energy, and under moderate momentum transfer to the target (~0.8 au), are presented. It is shown that, when the data are sorted according to the Bethe-ridge condition (i.e. the magnitude of the momentum transfer is equal to that of the sum momentum of the emitted electrons), the (e, 3e) angular distributions clearly bear the signature of the target initial two-electron momentum density, |ψ( p 1 , p 2 )| 2 . We show, for the first time in DI, the different signature due to the s or p character of the ionized electrons. For He, the results are qualitatively compared with a state-of-the-art first-order theory.