Longitudinal photon-momentum transfer in strong-field double ionization of argon atoms

We investigate the longitudinal photon-momentum transfer in nonsequential double ionization of argon atoms exposed to a $2.0\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ linearly polarized strong laser field. The zero momentum of the photoelectron is precisely calibrated using the photoelectron-photoion coincidence spectrum. The sum-longitudinal momentum of two photoelectrons from the double ionization noticeably deflects along the laser propagation direction, which is opposite to and much larger than that of single ionization. By tracking the electron trajectories for double ionization in semiclassical simulations, we find that the Coulomb focusing of the ionic core with the assistance of the Lorentz pushing of the laser field dominates the observed longitudinal momentum deflection. Our results open the possibility to control the photon-momentum transfer by manipulating the waveform of the laser fields and thus the Coulomb and magnetic effects experienced by the photoelectrons.