Collective tunnel ionization in atomic systems

Abstract

We present a theory of the collective tunnel effect in atomic systems subject to strong low-frequency laser fields. Using an analytic semi-classical method and numerical solution to the time-dependent Schrödinger equation we demonstrate the collective channel of the nonsequential double ionization in a model two-dimensional two-electron system with ionization potentials reproducing those of neutral xenon and of the negative bromine ion. Collective tunneling in the presence of the electron-electron repulsion comprises the joint sub-barrier motion of the two electrons along quantum trajectories transversely shifted in opposite directions with respect to the direction of the external electric field. Analytic results for the rates of collective tunneling allow indicating the intervals of laser intensity, where this effect can be expected to compete with sequential tunneling in helium, xenon, and negative bromine. Momentum distributions of the electron pairs carry clear signatures of the collective channel in the form of shoulders in the direction lateral to the field polarization.