Breakdown of the independent electron approximation in sequential double ionization

Correlated electron emission in strong field double ionization is dominated by recollision of the first ionized electron with its parent ion. With laser pulses that are close-to-circularly polarized, recollision may be greatly modified or avoided and the electrons are usually assumed to be field ionized without mutual interaction. Here, we present coincidence momentum measurements of the doubly charged ion and the two electrons, which are in disagreement with the independent electron assumption for close-to-circularly polarized fields. These experiments demonstrate that recollision is not the only mechanism that can lead to correlated electron emission in strong field double ionization.