Femtosecond CARS of Binary Liquid Mixtures: Dephasing via Concentration Fluctuations in the Intermediate Regime

The symmetric CH 3 -stretching mode ν 1 of CH 3 I is investigated in a solution of CDCl 3 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 distinguish the dephasing contribution of concentration fluctuations with correlation time >1 ps, indicating the intermediate case for this relaxation mechanism. The results are well described by the Knapp−Fischer model for concentration fluctuations, yielding a molecular residence time in the solvation layer of 1.1−3.2 ps in the temperature range 242−374 K in accordance with theoretical estimates.