Femtosecond CARS of binary liquid mixtures: Dephasing via concentration fluctuations in the intermediate regime

The symmetric CH3-stretching mode nu(1) of CH3I is investigated in a soluti on of CDCl3 in the concentration range 7-100% and temperature interval 242- 374 K using time-resolved coherent anti-Stokes Raman scattering (CARS) with 250 fs pulses and magic polarization geometries. Striking deviations from simple exponential relaxation dynamics are reported for mixtures with mole fractions around 0.5. Analysis of the concentration dependence allows to di stinguish the dephasing contribution of concentration fluctuations with cor relation time >1 ps, indicating the intermediate case for this relaxation m echanism. The results are well described by the Knapp-Fischer model for con centration fluctuations, yielding a molecular residence time in the solvati on layer of 1.1-3.2 ps in the temperature range 242-374 K in accordance wit h theoretical estimates.