LAUSR

The transfer of the linear photon momentum to the electron in the few-photon ionization of an atom is accurately computed through a numerical solution to the three-dimensional time-dependent Schr\"odinger equation (TDSE) beyond the dipole approximation. The characteristics of the transfer is studied in detail by comparing the TDSE results with those calculated by the perturbation theory (PT) using the exact scattering states. Our fully quantum-mechanical calculations show a discernible photoelectron momentum shift along the laser's propagation direction, which is caused by the nondipole effects. The momentum transfer rule for the single-photon ionization is shown to agree with results obtained from the perturbation theory. We identify some extraordinary ``dips'' in the photon-momentum transfer in the case of two-photon ionization, which depend on the laser ellipticity and are caused by the suppression of the nondipole transitions. In addition, the Coulomb screening effect on the photon-momentum transfer has also been investigated and we find that the Coulomb tail is negligible in the single-photon ionization case. However, the potential in the short range near the Coulomb center may influence the initial state, which will change the amount of the momentum that can be transferred.