Femtosecond stimulated Raman studies of vibrational coupling and ultrafast chemical reaction dynamics

I have used the unique spectroscopic capabilities of Femtosecond Stimulated Raman Spectroscopy (FSRS) to examine ultrafast chemical reaction dynamics with an emphasis on vibrational coupling. Using FSRS one can acquire Raman spectra with simultaneously high temporal (50 fs) and spectral (8 cm -1) resolution, thus obtaining structural snapshots throughout an ultrafast chemical reaction.;In initial studies, I examined what was thought to be ground state anharmonic coupling. By exciting wavepackets in the low frequency vibrational modes of CDCl3, sidebands with temporal oscillations were seen off of high frequency peaks in the FSRS spectrum. Later work probed the polarization dependence of these sidebands, proving that the symmetry of the coupled modes could be measured. In subsequent work on solvent mixtures I showed that these signals result from third-order cascades rather than the presumed fifth-order coupling signal.;In order to determine the origin of negative anti-Stokes and dispersive resonance features in FSRS, I examined the contribution of various Feynman pathways to the ground state FSRS signal from Rhodamine 6G. The changing dispersion of the resonance signals across the absorption band was explored, as well as the effects of the temporal overlap of the Raman pulses on competing processes.;In later work, I used FSRS to study excited state structural dynamics, including proton and electron transfer reactions. In green fluorescent protein, we monitored the excited state proton transfer which gives rise to its remarkable fluorescence, proving that a low frequency wagging mode gates the chemical reaction. This first experimental glimpse of the multidimensionality of a reaction coordinate paves the way for future work on mapping reactive potential energy surfaces.;My most recent work examined the vibrational modes which contribute to electron transfer, examining the interfacial electron transfer process from coumarin dye molecules to TiO2 nanoparticles. These studies were successful in identifying the modes which lead the complex out of the Franck-Condon region, as well as determining the localization of the hole on the coumarin backbone.;These experiments demonstrate the capabilities and versatility of the FSRS technique, and the importance of monitoring structure as a function of time as a means to understand chemical reactivity.