In strong laser fields (10~(14)-10~(15) W/cm~2, 800nm) all atoms have a high probability to become singly to become singly or even multiply ionized. These ionization processes can be studied in great detail by imaging the momenta of the resulting ions and electrons with COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy). The momenta of the ions give a direct insight in the role of electron correlation during the ionization process. They strongly support a scattering scenario in which at first one electron couples to the field and is finally driven back to its parent ion where it transfers part of its energy to the second electron. A double peak structure found in the ion momentum distribution gives a direct measure of the time delay introduced by the scattering trajectory. The two electrons are finally found to be emitted with similar momentum to the same hemisphere. These results are compared to double ionization by charged particle and single photon impact.
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