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