Two-dimensional, two-electron model atom in a laser pulse: Exact treatment, single-active-electron analysis, time-dependent density-functional theory, classical calculations, and nonsequential ionization

Owing to its numerical simplicity, a two-dimensional two-electron model atom. With each electron moving in one direction, is an ideal system to study nonperturbatively a fully correlated atom exposed to a laser field. Frequently made assumptions, such as the ''single-active-electron'' approach and calculational approximations:: e.g., time-dependent density-functional theory or (semi)classical techniques. can be tested. In this paper we examine the multiphoton short pulse regime. We observe ''nonsequential'' ionization, i.e., double ionization at lower field strengths as expected from a sequential, single-active-electron point of view. Since we also find nonsequential ionization ia purely classical simulations, we are able to clarify the mechanism behind this effect in terms of single-particle trajectories.