Time‐resolved anisotropic coherent anti‐Stokes Raman scattering: A new probe of reorientational dynamics

A formalism for the time‐dependent anisotropic third‐order susceptibility induced by a linearly polarized excitation pulse has been derived to describe the time dependence of coherent anti‐Stokes Raman scattered (CARS) and Raman time‐resolved experiments. The third‐order susceptibility induced in a randomly oriented molecular system contains nine independent molecular parameters—three isotropic and six anisotropic—as compared to three independent parameters in an isotropic system. Methods of time‐resolved anisotropic anti‐Stokes Raman scattering (TRA CARS) are discussed as an approach to reorientational measurements. The dependence of the TRA CARS or TRA Raman signal on vibration and polarization component can in principle provide more rotational information than fluorescence depolarization or linear dichroism measurements and may allow more complete characterization of rotational dynamics. Results are reported for three types of TRA CARS experiments to demonstrate the capabilities of TRA CARS to study the rotational motion of molecules in liquids.