  The transition between two distinct mechanisms for the laser-induced
field-free orientation of CO molecules is observed via measurements of
orientation revival times and subsequent comparison to theoretical
calculations. In the first mechanism, which we find responsible for the
orientation of CO up to peak intensities of 8 x 10^13 W/cm^2, the molecules are
impulsively oriented through the hyperpolarizability interaction. At higher
intensities, asymmetric depletion through orientation-selective ionization is
the dominant orienting mechanism. In addition to the clear identification of
the two regimes of orientation, we propose that careful measurements of the
onset of the orientation depletion mechanism as a function of the laser
intensity will provide a relatively simple route to calibrate absolute rates of
non-perturbative strong-field molecular ionization.
