Understanding and controlling electron-nuclear coupled dynamics in molecules
File(s)
Author(s)
Moore, Luke
Type
Thesis
Abstract
Recent advances in attoscience have shown that electron cloud dynamics in molecules, initiated by superpositions of electronic states, can influence nuclear motion and, by extension, their chemistry. Understanding these coupled electron–nuclear phenomena is vital towards understanding mechanisms in photobiology and light harvesting, as well as acting as a step forward in directing chemical reactivity through the control of these superpositions.
Despite significant progress in both experiment and theory, current understanding remains largely confined to small molecular systems. The mechanisms governing the formation and evolution of electronic state superpositions in larger, more complex molecules remain less certain.
This thesis aims to address this gap by employing the semi-classical second-order Ehrenfest (2OE) and fully quantum mechanical Quantum Ehrenfest (Qu-Eh) dynamics methods to investigate coupled electron–nuclear dynamics in medium-sized molecular systems. Pyrazine serves as the principal system of interest due to its well-characterized nonradiative decay from the bright ππ∗ state to the first excited singlet nπ∗ state via a conical intersection (CoIn); a process recently observed to involve electronic coherence.
With pyrazine, we study how electronic coherences play a role in the decay process. Our results support theoretical arguments that coherences do not form at a CoIn between states of different Abelian point group symmetries at the Franck-Condon region if excited to a pure state. However, we demonstrate that even a small state mixture (97:3) of the states involved in the CoIn can circumvent this restriction, enabling coherence formation. We also take an in-depth analysis of the effect of the electronic structure method used to model pyrazine on its excited state ordering, rationalizing this effect through the splitting of two pairs of nπ∗ states.
Finally, this thesis explores the numerical challenges behind the 2OE and Qu-Eh dynamics methods through simulation of molecular systems and suggest solutions and workarounds for future users.
Despite significant progress in both experiment and theory, current understanding remains largely confined to small molecular systems. The mechanisms governing the formation and evolution of electronic state superpositions in larger, more complex molecules remain less certain.
This thesis aims to address this gap by employing the semi-classical second-order Ehrenfest (2OE) and fully quantum mechanical Quantum Ehrenfest (Qu-Eh) dynamics methods to investigate coupled electron–nuclear dynamics in medium-sized molecular systems. Pyrazine serves as the principal system of interest due to its well-characterized nonradiative decay from the bright ππ∗ state to the first excited singlet nπ∗ state via a conical intersection (CoIn); a process recently observed to involve electronic coherence.
With pyrazine, we study how electronic coherences play a role in the decay process. Our results support theoretical arguments that coherences do not form at a CoIn between states of different Abelian point group symmetries at the Franck-Condon region if excited to a pure state. However, we demonstrate that even a small state mixture (97:3) of the states involved in the CoIn can circumvent this restriction, enabling coherence formation. We also take an in-depth analysis of the effect of the electronic structure method used to model pyrazine on its excited state ordering, rationalizing this effect through the splitting of two pairs of nπ∗ states.
Finally, this thesis explores the numerical challenges behind the 2OE and Qu-Eh dynamics methods through simulation of molecular systems and suggest solutions and workarounds for future users.
Version
Open Access
Date Issued
2025-12-16
Date Awarded
2026-03-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Bearpark, Michael
Robb, Michael
Sponsor
Engineering and Physical Sciences Research Council
Gaussian Inc. (Firm)
Grant Number
EP/T51780X/1
Publisher Department
Department of Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)