Strong-field approximation with leading-order nondipole corrections

Employing the nondipole Volkov states, i.e., the solutions of the time-dependent Schrödinger equation for an electron in a laser field whose spatial dependence is accounted for by including the first-order term in a 1/c expansion of the vector potential, we present an analytic approach based on the strong-field approximation that is able to describe ionization and detachment in intense midinfrared laser fields. Our approach includes both the direct transition amplitude and the improved strong-field approximation transition amplitude, accounting for the field-induced rescattering of the liberated electron on its parent core. We apply the theory to strong-field detachment from the F- ion. Comparing the results to those obtained invoking the dipole approximation, we identify and analyze the effects caused by including the nondipole corrections for the cases of circular and linear polarization. We demonstrate that the introduced nondipole corrections induce asymmetries in photoelectron momentum distributions along the propagation direction. For the case of linear polarization, we report a shift of the low-energy region of the momentum distribution against the propagation direction, a feature previously studied on neutral targets and related to Coulomb focusing effects.