Multi‐dimensional time‐resolved coherent Raman six‐wave mixing: a comparison of the direct and cascaded processes with femtosecond excitation and noisy light interferometry

The two‐dimensionally time‐resolved fifth‐order Raman spectroscopies potentially offer rich dynamic information about the condensed phase, yet these are often fraught by the competition with less revealing lower order events occurring in a cascaded manner. These competing processes have qualitatively similar time domain behavior, and in some cases the cascaded lower order effects may completely obscure the more interesting fifth‐order signal. This work seeks to characterize, through theoretical simulations, the true fifth‐order pseudo‐Raman echo and its various competing cascaded third‐order processes. Both homogeneous and inhomogeneous broadening are included in the material response functions in the standard fashion. Single and multiple modes, harmonic and (diagonally) anharmonic Raman oscillations are treated. Signals for fifth‐order femtosecond pulsed experiments are characterized for both homodyne and heterodyne detection. Likewise, the use of noisy light interferometry for two‐dimensional time resolution and discrimination among the competing processes is explored. In the case of a single driven Raman mode, discrimination among the competing processes becomes feasible only when inhomogeneous broadening is dominant. On the other hand, when two (or more) Raman coherences are created, the interesting true fifth‐order signal behaves quite differently in its two‐dimensional depiction than do the competing cascaded events. Copyright © 2000 John Wiley & Sons, Ltd.