Quasiclassical modelling of helium double photoionization

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. (author)