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We applied the method known as fermion molecular dynamics (FMD) to the description of a helium atom interacting with a short pulse of intense, long-wavelength laser radiation, for both linear and circular polarization. We describe the results of these calculations, insofar as they bear on the question of the mechanisms leading to double electron ejection. In the case of linear polarization, boomeranging trajectories leading to double ionization were observed at all laser intensities above threshold. The probability of occurrence was low, and almost independent of laser intensity. However, very near to threshold, boomeranging trajectories leading to double ionization were found to occur with a probability comparable to that for all other independent electron (sequential) processes. This produced a shoulder in the curve of double ionization probability versus laser intensity. The size of this shoulder was found to depend on laser wavelength and pulse length. No such trajectories were found for circular polarization.

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We describe the results of our quasiclassical calculations of the ionization probabilities of a helium atom, exposed to a short pulse of linearly polarized laser radiation. The technique known as Fermi molecular dynamics was applied. Of particular interest was our observation of the signature of a new process of double ionization, at relatively low laser irradiances. We interpret this as a signature of the simultaneous double ionization of helium. This new process was found to proceed in competition with sequential double ionization. The simultaneous double ionization of helium, as a distinct process at relatively long wavelengths, has just been discovered experimentally [

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