  Recollision processes provide direct insight into the structure and dynamics
of electronic wave functions. However, the strength of the process sets its
basic limitations - the interaction couples numerous degrees of freedom. In
this Letter we decouple the basic steps of the process and resolve the role of
the ionic potential which is at the heart of a broad range of strong field
phenomena. Specifically, we measure high harmonic generation from argon atoms.
By manipulating the polarization of the laser field we resolve the vectorial
properties of the interaction. Our study shows that the ionic core plays a
significant role in all steps of the interaction. In particular, Coulomb
focusing induces an angular deflection of the electrons before recombination. A
complete spatiospectral analysis reveals the influence of the potential on the
spatiotemporal properties of the emitted light.
