  We present measurements and calculations of the momentum distribution of
electrons emitted during the ion-atom collision 10 keV/u $He^{2+}+He
\rightarrow He^{+} + He^{2+} + e^{-}$, which show rich structures for ion
scattering angles above 2 mrad arising dominantly from two-electron states. Our
calculations reveal that minima in the measured distributions are zeros in the
electronic probability density resulting from vortices in the electronic
current.
