Laser-assisted and laser-induced electron-impact ionization during nonsequential double ionization of He

Two-electron sum-momentum distributions from nonsequential double ionization of He in an intense laser field are analyzed. The results of a S-matrix calculation show that the inelastic scattering of the first electron with the residual ion is best considered as a (internal) laser-induced impact ionization. The parallel component of the sum momentum is found to be largest when the prescattering electron energy is close to zero. This is due to the fact that a large momentum transfer between the sum momentum in the final state and the impact momentum in the intermediate state along the polarization axis favors the absorption of field energy by the two-electron system during the scattering process.