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 in purely classical simulations, we are able to clarify the mechanism behind this effect in terms of single-particle trajectories.