Infrared degenerate four wave mixing spectroscopy of jet-cooled C2H2

We report the application of infrared degenerate four-wave mixing (IR DFWM) spectroscopy to probe rotational-vibrational transitions in the v3/v2 + v4 + v5 Fermi dyad region of jet-cooled C2H2. The spectral intensities show a quadratic population dependence, and are well reproduced by a Boltzmann distribution of J state populations at Trot = 25 K. Intensity dependence measurements and lineshape analysis show a cubic dependence of the DFWM signal on laser intensity, as expected from theory in the weak field limit (I ⪡ Isat). We compare DFWM signals in jet versus bulk experiments.