  The resonant nonlinear refraction of 4-5 $\mu$m light in CO and CO$_2$ gas at
a peak intensity of 15 MW/cm$^2$ was demonstrated using time- and
frequency-resolved measurements of self-focusing and self-defocusing. The
nonlinearity of these molecular gases exhibits intensity-dependent sign
reversals and a < 4 ns response time. A change from self-focusing to
self-defocusing or vice-versa was observed to occur for Rabi frequencies that
are comparable to the collisional linewidth. A density matrix model for the
nonlinear susceptibility of a strongly driven two-level system provides a
qualitative explanation for these results.
