INFRARED DEGENERATE 4-WAVE-MIXING SPECTROSCOPY OF POLYATOMIC-MOLECULES - CH4 AND C2H2

We apply infrared degenerate four-wave mixing (IR DFWM) spectroscopy t o the investigation of CH4 and C2H2, making use of C-H stretching rovi brational transitions near 3000 and 3300 cm(-1), respectively. The spe ctrum of C2H2 is fitted quite well by a two-level stationary-absorber model in the low-saturation limit I<<I-sat 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 s imilar to 10 I-sat. The reason for the disagreement at low intensity i s not clear, but the results indicate that approximations made to acco unt for spectroscopic line broadening may be inadequate for the vs vib rational band of CH4. IR DFWM provides a sensitive diagnostic, similar to 10(11) molecules/cm(3) per quantum state, for the molecules invest igated here, and we believe that it holds great promise for the invest igation of polyatomic molecules that have IR active rovibrational tran sitions, in general.