Few-XUV-photon laser-assisted double ionization of helium

We studied the few--photon IR laser--assisted double ionization of helium in ultrashort XUV pulse(s) by numerically solving the time-dependent Schr{\"o}dinger equation in full dimensionality within a finite--element discrete--variable--representation scheme [1]. We calculated energy and joint angle distributions in coplanar geometry, where the emitted electron momenta and identical polarization axis of the linearly polarized XUV and IR pulses lie in a plane. By analyzing joint angle distributions and asymmetries for two-XUV--photon double ionization, we identify ``sequential'' and ``non-sequential'' contributions for ultrashort XUV pulses whose spectra overlap the sequential ($\hbar\omega>$ 54.4 eV) and non-sequential (39.5 eV $<\hbar\omega<$ 54.4 eV) double ionization regimes. In addition, we show that emission angles between the two photoelectrons can be controlled by adjusting parameters of the XUV and assisting IR pulse. \\[4pt] [1] A. Liu and U. Thumm, Phys. Rev. A \textbf{89}, 063423 (2014).