Femtosecond nonresonant degenerate four-wave mixing at atmospheric pressure and in a free jet

Time-resolved nonresonant degenerate four-wave mixing (DFWM) experiments on the femtosecond time scale were performed in the gas phase. Results are pr esented for air, O-2, N-2, and CO2. For CO2 additional experiments have bee n performed at reduced pressure and in a molecular beam. By delaying the pr obe pulse a periodic recovery of the DFWM signal is observed. The period of these transients can be assigned unambiguously to rotational Raman transit ions of the ground state within the laser bandwidth. The decay of the trans ients yields the collisional dephasing of the Raman-induced polarization. A t zero delay also optical-held-induced birefringence of electronic nature c ontributes to the signal. The different time scales of the Raman and electr onic effects allow us to estimate their relative strength.