COULOMB CORRECTION TO THE VOLKOV SOLUTION

A simple analytical approximation exists for the wave function of an u nbound electron interacting both with a strong, circularly polarized l aser field and an atomic Coulomb potential [1]. This wave function is the Volkov state with a first-order Coulomb correction coming from som e perturbative expansion of the potential in the Kramers-Henneberger r eference frame [2-4]. The expansion is valid if the distance from the center of the Coulomb force is smaller than the classical radius of mo tion of a free electron in a plane-wave field. We generalize this appr oach for any elliptic polarization. The total photoionization rate in the strong-field approximation grows more for linear than for circular polarization due to Coulomb correction.