IONIZATION OF HELIUM BY A SHORT-PULSE OF RADIATION - A FERMI MOLECULAR-DYNAMICS CALCULATION

We describe the results of our quasiclassical calculations of the ioni zation probabilities of a helium atom, exposed to a short pulse of lin early polarized laser radiation. The technique known as Fermi molecula r 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 simultaneo us double ionization of helium. This new process was found to proceed in competition with sequential double ionization. The simultaneous dou ble ionization of helium, as a distinct process at relatively long wav elengths, has just been discovered experimentally [D. Fittinghoff, P. Bolton, B. Chang, and K. Kulander, Phys. Rev, Lett. 69, 2642 (1993)].