| Summary: | The work in this dissertation probes the origins of spectral signatures in two-dimensional spectroscopy. Quantum beating signals have widely been used to inform the study of energy transfer in photosynthetic complexes. They arise from the coherent evolution of superpositions of eigenstates which modulates the signal as a function of time. Knowing which states are involved in the superpositions is imperative to understanding the dynamics.
Ultrafast, and therefore ultrabroadband, pulses will excite both vibrational and electronic coherences, and distinguishing between the two in 2D spectra can be challenging. Two-dimensional spectra of the laser dye Pyrromethene 650 are used to characterize vibrational quantum beats. The spectra show that vibrational coherences lead to quantum beating signatures throughout the spectra in both the rephasing and nonrephasing signals. Ubiquitous quantum beats are not expected for electronic coherences, and this distinguishing characteristic is discussed.
Long-lived quantum beats played an important role in determining the wave-like energy transfer mechanism of the Fenna-Matthews-Olson (FMO) antenna complex. This explanation depends on attributing the quantum beats to electronic coherences. In an attempt to identify the origins of quantum beating observed in FMO, 2D spectra of the bacteriochlorophyll a (Bchla) in solution characterize the possible vibrational coherences. The characteristics shown in the PM650 data are used to analyze the Bchla experiments. The quantum beats observed in the Bchla data are decidedly different than those observed in FMO. The assignment of the quantum beats in FMO to electronic coherences is discussed further.
A novel extension of 2D spectroscopy is proposed as a way to explore photoreactions where the coupling between nuclear and electronic modes is strong. The proposed fifth-order spectroscopy utilizes Raman interactions to explore the vibrational modes involved in such reactions. A detailed account of the spectroscopy is presented and its possible application to synthetic design is presented.
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