Time resolved four- and six-wave mixing in liquids .2. Experiments

Femtosecond four- and six-wave mixing is employed to study intermolecular motion in liquids, using CS2 as a working example. Nonresonant four-wave mixing yields the total spectral response associated with the low-frequency motions in the liquid. The results of optical Kerr effect and transient grating scattering experiments can be modeled equally well by homogeneously and inhomogeneously broadened intermolecular vibrations. Femtosecond nonresonant six-wave mixing, where two independent propagation times can be varied, contains a temporally two-dimensional contribution that provides information on the time scale(s) of these intermolecular dynamics. The six-wave mixing signal of CS2 shows distinctly different behavior along the two time variables. When the first propagation time is varied, both librational motion at short times and a picosecond diffusive tail are observed. Along the second propagation time, there is no sign of diffusive response and the signal is solely determined by the librational motions. Its shape depends on the first propagation time, when it is varied between 0 and 500 fs, but it is unaffected by further increase of that delay. This is a strong indication for a finite correlation time of the fluctuations in the intermolecular potentials. The interplay between the initial coherent motions and the diffusive behavior on longer time scales is far from clear. A widely used model in which these are treated as independent harmonic processes fails to describe the results. (C) 1997 American Institute of Physics.