Infrared degenerate four-wave mixing spectroscopy of polyatomic molecules: CH_4 and C_2H_2

We apply infrared degenerate four-wave mixing (IR DFWM) spectroscopy to the investigation of CH4 and C2H2, making use of C—H stretching rovibrational transitions near 3000 and 3300 cm−1, respectively. The spectrum of C2H2 is fitted quite well by a two-level stationary-absorber model in the low-saturation limit, I ≪ Isat. The spectrum of CH4 is not accurately described by this model in the low-saturation limit, but it is described well by the same model in the high-saturation limit, I ∼ 10 Isat. The reason for the disagreement at low intensity is not clear, but the results indicate that approximations made to account for spectroscopic line broadening may be inadequate for the ν3 vibrational band of CH4. IR DFWM provides a sensitive diagnostic, ∼10 molecules/cm3 per quantum state, for the molecules investigated here, and we believe that it holds great promise for the investigation of polyatomic molecules that have IR active rovibrational transitions, in general.