Ionization of helium by a short pulse of radiation: A Fermi molecular-dynamics calculation

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 [D. Fittinghoff, P. Bolton, B. Chang, and K. Kulander, Phys. Rev. Lett. 69, 2642 (1993)].