Quasiclassical modelling of helium double photoionization

We applied the method known as fermion molecular dynamics (FMD) to the desc ription of a helium atom interacting with a short pulse of intense, long-wa velength laser radiation, for both linear and circular polarization. We des cribe the results of these calculations, insofar as they bear on the questi on of the mechanisms leading to double electron ejection. In the case of li near polarization, boomeranging trajectories leading to double ionization w ere observed at all laser intensities above threshold. The probability of o ccurrence was low and almost independent of laser intensity. However, very near to threshold, boomeranging trajectories leading to double ionization w ere found to occur with a probability comparable to that for all other inde pendent electron (sequential) processes. This produced a shoulder in the cu rve of double ionization probability versus laser intensity. The size of th is shoulder was found to depend on laser wavelength and pulse length. No su ch trajectories were found for circular polarization.