Mechanism for high-energy electrons in nonsequential double ionization below the recollision-excitation threshold

High-energy cutoffs for individual electrons from nonsequential double ionization of helium by a 390 nm laser pulse at intensity 0.8 PW/cm(2) have been investigated using a classical 3D ensemble. Below the recollision-excitation threshold, both electrons are bound after recollision and get ionized one by one later. With the trajectory back analyzing of electrons with final energies above 2U(p), we find an additional mechanism for the latter ionized electron after recollision. Both nuclear and laser forces contribute in this mechanism which leads to a broad transverse momentum distribution.