Kirkby, OM; (2015) The role of πσ* states in the ultrafast non-radiative dynamics of pyrrole, aniline and phenol. Doctoral thesis , UCL (University College London).

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Abstract

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.

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