Two-Dimensional Raman - Two-Dimensional Electronic Spectroscopy: From Isolating Ground-State Vibrational Coherences to Exciton-Phonon Spectroscopy of Quantum Dots

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Ultrafast, multi-dimensional coherent spectroscopy (MD-CS) has enabled scientists to probe fundamental aspects of chemical and photo-physical reactions on the nanosecond to femtosecond timescales. Using MD-CS, scientific contributions ranging from increased understanding of energy transfer in photosynthetic biological proteins to correlating the motion of electrons in semiconductors have been achieved. Concurrent with these (and many other) discoveries, the development of increased experimental sophistication and instrumental sensitivity allowed for information about these processes to be derived more rapidly and comprehensively, with the following goal always in mind: that no level of physical or dynamical understanding be obscured or ambiguous due to limitations of the apparatus. However, despite the tremendous gains in chemical knowledge and technical expertise, third-order MD-CS still suffers from spectral congestion and uncertain spectral assignment, especially as it concerns the convolution of electronic and vibrational degrees of freedom. The first part of this thesis is centered around the development and implementation of a novel fifth-order MD-CS technique, known as Gradient-Assisted Multi-dimensional Electronic-Raman Spectroscopy (GAMERS), designed specifically to address and alleviate the aforementioned problems. Due to the careful temporal ordering of resonant and non-resonant excitation pulses, this technique allows for vibrational coherences on the ground-state to be isolated from that of the excited-states. The viability of GAMERS is demonstrated on two laser dyes, IR-895 and IR-140.', 'In general, the more dimensions introduced into a MD-CS technique, the better the spectral resolution and discrimination, lending towards increased information being derived about the system under study. However, the price paid for increased dimensionality is exponentially increased data acquisition times, placing stress on apparatus stability and sample integrity, which, ultimately, inhibits practical implementation of higher order MD-CS techniques. The second part of this thesis deals with a methodological advancement of GAMERS, where non-uniform sampling based on the projection-slice theorem and the inverse Radon transform significantly increases the speed of signal acquisition. In addition, we demonstrate that this method, denoted PROjection-reconstruction GAMERS (PRO-GAMERS), as compared with uniform sampling, has superior sensitivity and does not significantly sacrifice frequency resolution. These advantages are a consequence of our non-uniform sampling method in general, and would apply to any high dimensionality MD-CS technique. We demonstrate the utility of PRO-GAMERS on Nile Blue A perchlorate.', 'Quantum Dots (QDs) have shown promise for a range of practical applications, due to the wide synthetic tunability of their absorption and emission spectra. Despite an array of synthetic and spectroscopic studies, a full understanding of the underlying electronic structure has been difficult to realize. To achieve this, it is not only necessary to separate homogeneous peak shapes from heterogeneous ones, but to also separate overlapping homogeneous features from each other, a feat that has only been realized under certain experimental conditions. In the last part of this thesis, we use GAMERS to study colloidal CdSe QDs at room temperature in solution, separating the signal across two dimensions involving excitonic transitions and one involving phonon energies. In effect, we not only demonstrate the ability of GAMERS to resolve the congested spectral peak shapes to the extent aforementioned, thereby approaching the single-particle linewidth, but we also collect statistics regarding the ensembles present within the sample, all done under sample conditions representative of those wherein QDs are applied practically (i.e., room temperature). This opens a space for novel scientific inquiries, as GAMERS unlocks the ability to grasp single-particle electronic structure for a wide variety of complex systems where room temperature dynamics are most relevant.

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  • 09/30/2019
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