Mechanisms of double ionization in strong laser field from simulation with Coupled Coherent States: Beyond reduced dimensionality models

The recently developed Coupled Coherent States method for solution of the time-dependent Schrödinger equation is used to simulate the strong laser field double ionization of a helium atom in six spatial dimensions. The calculated double ionization yield, as a function of laser intensity, reproduces the “shoulder”, attributable to electron recollisions. The method, which employs a Coherent State representation guided by classical trajectories also provides physical insight into the ionization mechanism. The appearance of the 'shoulder', is seen by analysis of the guiding trajectories to arise predominantly from sequential excitation of one electron into a highly excited orbit, followed by an electron-electron collision leading to correlated escape of the two electrons. A second little known mechanism involves recollisions resulting in ionization of the secondary electron, leaving the first in a highly excited state, from which it is later ionized by the laser field.