Line shapes of resonant features in four-wave mixing (FWM) spectra are modified by interferences between the electric fields produced by resonant and nonresonant nonlinear optical processes. In experiments on liquid samples, the windows of the optical cell can make a significant nonresonant contribution, and this must be treated correctly when modeling FWM spectra. A model is presented that fully treats the interference between sample and window contributions to FWM signals. A laminar flow cell, which eliminates windows from FWM experiments on liquids, was also demonstrated. Optical hyperpolarizabilities and susceptibilities were measured for FWM processes in liquid acetonitrile, liquid water, and borosilicate glass. A relatively small nonlinearity was found for nonresonant processes in water (γ = 8.6 X 10-<sup>38</sup> cm<sup>6</sup>/erg), which would make it a useful solvent for vibrationally enhanced nonlinear spectroscopy. Experiments were performed to observe singly vibrationally enhanced (SIVE) processes associated with the fundamental stretch vibrations of water. No SIVE signal was detected, and an upper bound of γ < 3 X 10<sup>-37</sup> cm<sup>6</sup>/erg was determined for its nonlinearity.
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