Population relaxation and non-Markovian frequency fluctuations in third- and fifth-order Raman scattering

The third- and fifth-order Raman response is calculated for vibrational phase relaxation due to stochastic Gaussian frequency fluctuations. This model can be solved exactly using the cumulant expansion technique. It interpolates continuously between the homogeneous and inhomogeneous limit and allows for a description of non-Markovian dynamics arising from finite correlation time(s) of the frequency fluctuations. Model calculations demonstrate that temporally two-dimensional fifth-order Raman experiments are sensitive to the time scale(s) of these frequency fluctuations and also to the life times of the vibrational states. These processes, that cannot be distinguished in nonresonant third-order Raman scattering, can be unequivocally identified in fifth-order experiments. (C) 1998 Elsevier Science B.V. All rights reserved.