  Molecules show a much increased multiple ionization rate in a strong laser
field as compared to atoms of similar ionization energy. A widely accepted
model attributes this to the action of the joint fields of the adjacent ionic
core and the laser on its neighbor inside the same molecule. The underlying
physical picture for the enhanced ionization is that the up-field atom that
gets ionized. However, this is still debated and remains unproven. Here we
report an experimental verification of this long-standing prediction. This is
accomplished by probing the two-site double ionization of ArXe, where the
instantaneous field direction at the moment of electron release and the
emission direction of the correlated ionizing center are measured by detecting
the recoil sum- and relative-momenta of the fragment ions. Our results
unambiguously prove the intuitive picture of the enhanced multielectron
dissociative ionization of molecules and clarify a long-standing controversy.
