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Two-Dimensional Electronic-Vibrational Spectroscopy

  • Author(s): Lewis, Nicholas Henry Cohen
  • Advisor(s): Fleming, Graham R
  • et al.

This dissertation describes the development and initial applications of Two-Dimensional Electronic-Vibrational spectroscopy (2DEV), a multidimensional ultrafast spectroscopic technique that is capable of directly probing the coupling between electronic and vibrational transitions in molecular species.

This thesis first provides the necessary background and motivation driving the development of this spectroscopy, discussing the role of electronic-vibrational coupling in small molecule photophysics and the ways we can take advantage of these physics to study electronic excitation transfer in photosynthetic light harvesting systems. It then discusses the details of the apparatus constructed to perform the 2DEV experiments, as well as some of the important aspects of the signal processing and extraction of the desired signal, using phase cycling techniques. It goes on to describe the development of a simple model to be used in understanding the new information content accessible with 2DEV, and presents experimental results from the laser dye DTTCI. The basic predictions of the model, such as the dynamics of the spectral lineshape, are shown to hold in these experimental results, and we provide an experimental measurement for the difference in the strength of the vibration-bath coupling between the electronic ground and excited states.

In chapter 4, the model is extended to the case of an electronically coupled dimer, and a method is proposed by which 2DEV could potentially be used to study the movement of electronic excitations through a molecular aggregate, such as a photosynthetic light harvesting complex. It is shown how it might be possible to measure the electronic excited state populations directly in the site basis, without the need for an accompanying model. To demonstrate an experimental implementation of this method, the 2DEV spectra of Chlorophyll a and b in solution are then presented, assigning the important spectral features and demonstrating conclusively the connection between spectral shifts along the electronic and vibrational axes due to changes in the axial coordination state of the central Mg. This is a first step which is then expanded upon with the demonstration of the experimental application of 2DEV to the major light harvesting complex in plants, LHCII, and an illustration that the technique proposed in chapter 4 is technically sound. Indications are observed of long lived population of Chl b which contradicts the predictions of current models for the excitonic structure of this system, and calls into question our current understanding of the initial energy transfer dynamics in plant photosynthesis.

Finally, a discussion is provided of some of the promising directions for 2DEV to be further developed in the future.

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