Relaxation parameters via femtosecond CARS in multimode molecular systems

A time-dependent description of the third-order polarization of nonlinear multimode electronic-vibrational systems is proposed, which is based on the method of the transform theory. In this method, the multimode information is automatically transmitted through the calculation via the measured absorption, whereby only the parameters of the Raman-active mode that contributes in the CARS process are extracted. By varying the excitation frequencies in the CARS experiment, different modes can be investigated. Up to now, the transform laws between the absorption and the Raman scattering exist only for the case of monochromatic (stationary) excitation. Here the case of resonant (with an electronic transition) excitation with three short pulses is examined. The expression for the third-order polarization, which corresponds to the CARS signal, is derived via the absorption by using the time-dependent representation. The general statements are illustrated by examining a model which takes into account a local mode with bulk modes.