  Photoelectron momentum distributions (PMDs) from atoms and molecules undergo
qualitative changes as laser parameters are varied. We present a model to
interpret the shape of the PMDs. The electron's motion is guided by a
fictitious particle in our model, clearly characterizing two distinct dynamical
behaviors: direct ionization and rescattering. As laser ellipticity is varied,
our model reproduces the bifurcation in the PMDs seen in experiments.
