High-order above-threshold ionization beyond the electric dipole approximation: Dependence on the atomic and molecular structure

We provide an in-depth analysis of high-order above-threshold ionization of atoms and molecules by strong laser pulses, using three different theoretical approaches beyond the electric dipole approximation: (i) the numerical solution of the time-dependent Schrodinger equation, (ii) the classical three-step model, and (iii) the quantum-orbit model. In the classical and quantum-orbit models, we include an accurate description of the rescattering step by target-specific differential scattering cross sections. The considerable forward shift of the photoelectron momentum distributions along the laser propagation direction can be understood in terms of the nondipole electron motion after rescattering. An explanation of the additionally observed forward-backward asymmetry of the signal strength requires the accurate modeling of the rescattering step and the electron dynamics before the rescattering event. For the H-2(+) molecular ion, we compare the cases of parallel and perpendicular alignment of the molecular axis and we show that the interference pattern and its modification due to the nondipole effects are orientation dependent. Compared to atoms, the nondipole effects in molecular high-order above-threshold ionization appear more pronounced and amenable to experimental observation.