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The role of πσ* states in the ultrafast non-radiative dynamics of pyrrole, aniline and phenol

This thesis comprises three time-resolved photoelectron imaging (TRPEI) studies, examining the ultraviolet (UV) photorelaxation dynamics of molecules with low-lying πσ* states. Chapter 1 introduces important principles in molecular photochemistry, wavepacket dynamics and the relaxation mechanisms enabled by these πσ* states. Chapter 2 describes the experimental apparatus and data analysis techniques used to extract information from raw TRPEI spectra. Chapters 3-5 contain the experimental data. Chapter 3 examines the UV relaxation dynamics of pyrrole, where the lowest excited singlet state is of πσ* character. We observe a single sub-50 fs, dissociative decay for all wavelengths studied, in keeping with the literature. The pyrrole dimer is used as a model for microsolvation, where a new decay pathway opens, relating to relaxation into stabilised charge transfer states. Chapter 4 details the experimental study of aniline. Direct excitation to the S2 with various excess vibrational energies, shows that the S2/S1 conical intersection is located close to the S2(πσ*) origin and enables the dominant S2 → S1 internal conversion mechanism. Population on the S1(ππ*) surface (following relaxation or direct excitation) undergoes considerably slower internal conversion to the ground state, the invariance of this decay in deuterated aniline suggests that any S1 → S2 tunnelling is not favoured. Chapter 5 studies the UV photodynamics of phenol, finding that internal conversion to the ground state is the dominant relaxation mechanism in the S1(ππ*) state, in agreement with the literature. Increasing the excitation energy to access to the S2(πσ*) state opens a minority pathway to O-H dissociation, but IVR on the S1 surface followed by internal conversion continues to dominate. Finally, Chapter 6 summarises the surprising variety in the mechanisms that πσ* states facilitate, even in these closely related molecules, and provides an outlook for future work.
Date January 2015
CreatorsKirkby, O. M.
PublisherUniversity College London (University of London)
Source SetsEthos UK
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation

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