Nondipole effects in helium ionization by intense soft x-ray laser pulses

We consider the helium atom exposed to laser pulses in soft-x regime at intensities of the order of 10(17)-10(20) W/cm(2). Two cases are investigated, first a pulse with a central frequency of 20 a.u. (similar to 522.2eV) and a pulse with a central frequency of 50 a.u. (similar to 1.36keV). We solve the time-dependent Schrodinger equation (TDSE), nondipole (retardation) effects are included up to O(1/c) (c being the velocity of light). We study the single photoionization of helium with a focus on nondipole effects. We calculate the total and partial photoelectron energy spectra, and the photoelectron angular distributions. The nondipole effects associated with the AP and A(2) coupling terms in the Hamiltonian (A denoting the vector potential of the field and P the momentum operator) are thoroughly analyzed in the region of one-photon resonance. We show that the nondipole contribution associated with A<bold>P varies linearly with the intensity I while a three</bold>-photon transition involving A(2) increases like I-3, in agreement with lowest order perturbation theory (LOPT). At high intensities these contributions compete and the nondipole transition becomes nonlinear. The physical mechanisms associated with nondipole couplings are discussed, as well as their effects on photoelectron energy and angular distributions.