Modelling ultrafast two-dimensional spectroscopy of vibronic systems using non-Markovian hierarchical equations of motion.

Green, Dale (2020) Modelling ultrafast two-dimensional spectroscopy of vibronic systems using non-Markovian hierarchical equations of motion. Doctoral thesis, University of East Anglia.

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Two-dimensional spectroscopy utilises a series of ultrafast optical interactions to create excited populations and track the decay of resulting wavepackets. This enables the study of the potential energy surfaces of complex chemical and biological systems, including the rates of relaxation between states and the dephasing of ensembles. But the inherent complexity of the condensed phase, associated with the vast degrees of freedom and disorder, presents significant challenges in modelling such photophysical processes. In particular, the similarity in relaxation timescale of the system and its surrounding environment provides the opportunity for feedback of information, introducing memory effects which have a major impact on the spectral lineshape. The shape and duration of the applied laser pulses also leads to filtering effects, such that spectra of complex systems can easily be misinterpreted. In this research, theoretical models for the simulation of two-dimensional electronic spectroscopy of vibronic systems are developed in both the underdamped and overdamped limits, using the hierarchical equations of motion to account for non-Markovian memory effects. Firstly, an investigation into the origins of spectral broadening from the perspective of quantum information theory finds that underdamped environments involve greater non-Markovian effects, but also that increased inhomogeneous broadening in overdamped environments is correlated with greater measurable non-Markovianity. The role of the laser spectrum is then demonstrated through spectral filtering of the coherence pathways of a vibronic zinc-porphyrin monomer. Changes in the 2D spectra on formation of delocalized exciton states in vibronic dimers are then examined in terms of a series of perylene bisimide homodimers, where the electronic coupling is controlled by increasing the monomer separation distance. Finally, an analysis of vibrational relaxation within a vibronic heterodimer, combined with selective laser excitation, demonstrates the full capability of the model by simulating energy transfer within an excitonic aggregate involving both system and environmental vibrational motion.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Chemistry
Depositing User: Chris White
Date Deposited: 15 Apr 2021 11:29
Last Modified: 15 Apr 2021 11:29


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