Classical threshold behaviour in a (1+1)-dimensional model for double ionization in strong fields

Abstract

Building on insights into the classical pathways to double ionization in strong laser fields, we here propose a (1+1)-dimensional model that captures essentials of the 3D potential landscape and allows efficient studies of the process in reduced dimensions. The reduction to one degree of freedom for each electron is obtained by confining their motion to lines at an angle of π/6 with respect to the field axis; the justification for this choice is that upon variation of the electric field the Stark saddles move along the same lines. In this way, we obtain a low-dimensional representation in which symmetric electron escape is possible. As a demonstration of the advantages of this model, we numerically confirm the equivalent of the Wannier threshold behaviour for constant electric field and identify several classes of trajectories that can contribute to the ionization process.