Collective tunnel ionization in atomic systems
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 Schrodinger equation we demonstrate the collective channel of the nonsequential double ionization in neutral xenon and in 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. 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. We propose and discuss potential experimental approaches to a search for collective tunneling in atomic systems using short circularly polarized laser pulses.