Strong field double ionization: what is under the 'knee'?

Both uncorrelated ('sequential') and correlated ('nonsequential') processes contribute to the double ionization of the helium atom in strong laser pulses. The double ionization probability has a characteristic 'knee' shape as a function of the intensity of the pulse. We investigate the phase-space dynamics of this system, specifically by finding the dynamical structures that regulate the ionization processes. The emerging picture complements the recollision scenario by clarifying the distinct roles played by the recolliding and core electrons. Our analysis leads to verifiable predictions of the intensities where qualitative changes in ionization occur, leading to the hallmark 'knee' shape.