We use time- and frequency-resolved measurements of self-focusing and self-defocusing of 4.3-μm radiation in CO2 gas to study the resonant nonlinear optical response in the vicinity of individual rovibrational lines. Measurements over a range of lines on the 4P and 4R branch of the 000-001 transition indicate that the nonlinear response of CO2 is strongly affected by power broadening, resulting in a sign reversal of the nonlinearity as compared to that of a saturable two-level system.
