Correlated electron emission in multiphoton double ionization

Electronic correlations govern the dynamics of many phenomena in nature, su ch as chemical reactions and solid state effects, including superconductivi ty. Such correlation effects can be most clearly investigated in processes involving single atoms. In particular, the emission of two electrons from a n atom-induced by the impact of a single photon(1), a charged particle(2) o r by a short laser pulse(3)-has become the standard process for studies of dynamical electron correlations. Atoms and molecules exposed to laser field s that are comparable in intensity to the nuclear fields have extremely hig h probabilities for double ionization(4,5); this has been attributed to ele ctron-electron interaction(3). Here we report a strong correlation between the magnitude and the direction of the momentum of two electrons that are e mitted from an argon atom, driven by a femtosecond laser pulse (at 38 TW cm (-2)). Increasing the laser intensity causes the momentum correlation betwe en the electrons to be lost, implying that a transition in the laser-atom c oupling mechanism takes place.