Resolving the emission times of solute and solvent four-wave mixing signals by spectral interferometry.

Electric field-resolved transient grating measurements are used to distinguish the four-wave mixing signal emission from a resonant solute and a non-resonant solvent. The two components of the solution (i.e., solute and solvent) emit signal fields at different times with respect to the arrival of the probe pulse to the sample. This gives rise to a recurrence in the temporal profile of the total signal field. We show that the origin of this interference is the difference in relaxation time scales of the holographic gratings associated with the solute and solvent. The grating of the resonant solute relaxes on the time scale of a few picoseconds due to depopulation of its excited electronic state, whereas the electronic polarizability response of the solvent relaxes on the femtosecond time scale. This separability of responses is a general phenomenon that is particularly useful for studying weakly absorbing solute dynamics in polarizable solvents.