Virtual Sequential Picture for Nonsequential Two-Photon Double Ionization of Helium

By using a model based on the second-order time-dependent perturbation theory, we show that the nonsequential two-photon double ionization of He can be understood in a virtual sequential picture: to excite the final double continuum state vertical bar k(1); k(2)> by absorbing two photons from the ground state vertical bar 1s(2); S-1(0)>, the single continuum states vertical bar 1s; k(1)> and vertical bar 1s; k(2)> serve as the dominant intermediate states. This virtual sequential picture is verified by the perfect agreement of the total ionization cross section, respectively, calculated by this model and by the sophisticated numerical solution to the full-dimensional time-dependent Schrodinger equation. This model, without the consideration of the electron correlation in the final double continuum state, works well for a wide range of laser parameters extending from the nonsequential to the sequential regime. The present Letter demonstrates that the electron correlation in the final double continuum state is not important in evaluating the total cross section, while it is indispensable for an accurate computation of a triply differential cross section. In addition, the virtual sequential picture bridges the sequential and nonsequential two-photon double ionization and reveals connections and distinctions between them.National Natural Science Foundation of China (NNSFC) [11322437]; National Program on Key Basic Research Project (973 Program) [2013CB922402]; NNSFC [11174016, 11121091]; Program for New Century Excellent Talents in University (NCET)SCI(E)EIPubMedARTICLEliangyou.peng@pku.edu.cn1515300211