Multiphoton above-threshold ionization in superintense free-electron x-ray laser fields: Beyond the dipole approximation

Abstract

We present an accurate and efficient nondipole computational approach in momentum space for the nonperturbative study of multiphoton above-threshold ionization (ATI) of atoms in superintense and ultrashort-wavelength laser fields. This approach has been successfully used to investigate the multiphoton processes of a hydrogen atom exposed to superintense free-electron x-ray laser fields. We find that, compared to results of the dipole approximation, the nondipole ATI spectra are enhanced substantially in the high-energy regime, and the photoelectron angular distributions are distorted significantly in higher-intensity and/or longer-pulse laser fields; in particular two lobes are induced, one along and one against the laser propagation direction. The origin of these phenomena has been explored in detail.