Laser-induced non-sequential double ionization investigated at and below thethreshold for electron impact ionization

Abstract

We use correlated electron–ion momentum measurements to investigatelaser-induced non-sequential double ionization of Ar and Ne. Light intensitiesare chosen in a regime at and below the threshold where, within therescattering model, electron impact ionization of the singly charged ioncore is expected to become energetically forbidden. Yet we find Ar2+ion momentum distributions and an electron–electron momentum correlationindicative of direct impact ionization. Within the quasistatic model this may beunderstood by assuming that the electric field of the light wave reduces theionization potential of the singly charged ion core at the instant of scattering. Thewidth of the projection of the ion momentum distribution onto an axisperpendicular to the light beam polarization vector is found to scale with thesquare root of the peak electric field strength in the light pulse. A scaling like thisis not expected from the phase space available after electron impact ionization. Itmay indicate that the electric field at the instant of scattering is usually differentfrom zero and determines the transverse momentum distribution. A comparison ofour experimental results with several theoretical results is given.