| Understanding anharmonic vibrational coupling in molecules is important for understanding heat dissipation on the microscopic scale. In Raman spectroscopy, anharmonic coupling can be measured by fifth-order interaction, but this signal is overwhelmed by two third-order interactions, or cascades. This thesis describes two attempts at attenuating the cascade signal in two-dimensional femtosecond stimulated Raman spectroscopy (2D-FSRS). Theoretically, changing phase-matching conditions and sample concentration were predicted to attenuate the third-order cascade and to thereby allow observation of the fifth-order signal. Experimentally, both methods were found to decrease the cascade, however no fifth-order signal was apparent.;The next method to measure anharmonic coupling was termed seventh-order Raman. In this implementation, seventh-order Raman uses a method called chirped-pulse stimulated Raman spectroscopy (CP-SRS) to create excited population in a low- frequency vibrational mode. When the low-frequency mode is excited, high frequency modes will shift frequencies according to their anharmonic coupling to the excited mode. CP-SRS is a new technique that is characterized and modeled in this thesis. This method uses two duplicates of a chirped, broadband laser pulse. When the pulses are delayed relative to each other, a narrow bandwidth instantaneous frequency difference is created that can excite Raman active vibrations in the sample via a stimulated Raman process. Currently, CP-SRS only excites approximately 0.05% of the molecules in the sample and the shift in the high frequency modes is too small to measure.;With the growth in available stimulated Raman methods, it is important to compare the various benefits associated with each. This thesis provides a comparison of three different methods to obtain a stimulated Raman spectrum; FSRS, CP-SRS and broad-band impulsive vibrational spectroscopy (BB-IVS). These three methods are all two-pulse experiments, but have different combinations of femtosecond and picosecond pulses and use either the time or frequency domain for probing. The advantages and disadvantages of the three methods are discussed in the comparison. |
