Exploration of strong-field multiphoton double ionization, rescattering, and electron angular distribution of He atoms in intense long-wavelength laser fields: The coupled coherent-state approach

We extend the coupled coherent-state (CCS) approach to simulate the strong-field ionization of helium atoms at long wavelengths. This approach uses a basis of trajectories guided by frozen Gaussian coherent states, sampled from a Monte Carlo distribution, as the initial states of the quantum time-dependent Schrodinger equations. The CCS trajectories move over averaged potentials, which can remove the Columbic singularities exactly. The low-energy structure is predicted by our CCS calculation and a rescattering event is clearly identified in the higher-energy regime. In addition, the nonsequential double ionization is also explored and the rescattering event can be identified as the major mechanism. Finally, we also study the electron angular distribution of helium. It is found that the maximum angle between the electron and electric field directions becomes smaller with increase in the laser intensity and wavelength.