Approximation for a Coulomb-Volkov solution in strong fields

A simple analytical approximation is found for the wave function of an electron simultaneously exposed to a strong, circularly polarized plane-wave field and an atomic Coulomb potential. The approximation is valid when ${\mathrm{\alpha }}_0$≫1, where ${\mathrm{\alpha }}_0$ is the classical radius of motion of a free electron in the plane-wave field. This constraint is sufficiently mild at low frequencies that it makes possible a major extension of the lower bound of laser intensities for which Volkov-solution-based approximations are useful.