Abstract
A neutral atom interacting with a sufficiently strong laser field will ionize even for photon energies well below the ionization potential. In contrast to the photoelectric effect, where a single photon of sufficient energy liberates an electron from the atom, multiphoton ionization (MPI) can occur for arbitrarily low photon energies provided that the intensity of the light is high enough. Since 1965 when MPI was first observed [1] it has grown into an active field in atomic and molecular physics. Single electron ionization has been extensively studied and has resulted in a comprehensive theoretical understanding [2-4]. However, the simultaneous ionization of two or more electron cannot be modeled simply as an extension of single ionization [5,6], Numerous experiments have shown that atoms exposed to intense laser fields have a tendency to liberate two or more electrons more often than expected [3,6,7]. Due to its simplicity, however the study of helium has helped solve part of the puzzle of how very intense visible light doubly ionizes atoms [8-10]. These experiments suggest that rescattering of the first ionized electron with the atom may ease the removal of the second electron; explaining the measured helium double ionization rates [11]. The same model can also explain the observed double ionization electron energy distributions.
© 2001 Optical Society of America
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