| In recent years, a great deal of progress has been made in understanding the ionization processes that result from the interaction of an intense laser pulse with multielectron atoms. However, due to experimental limitations, the effect of the laser field's spatial dependence on strong-field processes has rarely been investigated. Presented in this work is a theoretical analysis of this spatial dependence including a proposal for an experimentally observable result of the phenomenon. We begin by outlining the elements of the laser field that will vary as a function of position and show their effects on simple free electron trajectories. We then develop a classical, three-dimensional simulation of the entire process of double ionization of helium in an intense laser field using realistic, non-paraxial focal conditions. The existence of an out-of-phase electric field component in the laser propagation direction is determined, which produces an effective longitudinal ellipticity, resulting in a reduction in the double ion yields as a function of position in the laser focus. It is found that under conditions of tight focusing, the effective focal volume for non-sequential double ionization is significantly reduced. |
