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Title: Towards time-resolved X-ray absorption spectroscopy in organic polymers
Author: Wood, David
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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This thesis details developments towards the long-standing goal of time-resolved X-ray spectroscopy with laboratory-scale sources in order to study ultrafast dynamics in complex molecular and solid state systems. To this end, we present the construction of a table-top soft X-ray (SXR) source based on high harmonic generation (HHG), which delivers sub-femtosecond (i.e. attosecond, 1 as = 10−18 s) duration pulses with photon energies spanning the entire water-window region of the electromagnetic spectrum (284 eV - 540 eV). The photon flux and conversion efficiency of this source outperforms those previously reported by an order of magnitude, with simulations suggesting this improved performance results from entering a new, over-driven regime, providing an important roadmap for the development of next generation bright SXR sources. The suitability of this source for X-ray absorption spectroscopy is demonstrated by per- forming X-ray absorption near edge structure (XANES) measurements in poly(3- hexylthiophene) (P3HT), a semiconducting polymer used in organic photovoltaic devices which shows great promise as potential source of green, inexpensive, and renewable electrical energy. The earliest stages (≤ 500 fs) following photoexcitation in P3HT were studied by incorporating this source into a pump-probe experiment, and we present experimental data which tracks the evolution of the system with few femtosecond resolution. A pressing question concerning photoharvesting systems is whether quantum coherence plays a role in efficient energy transfer; our results provide evidence for coherent superposition-driven dynamics in P3HT, although further investigation is required. In parallel, work was done to extend HHG spectroscopy to polyatomic molecules. We present the development of an experimental apparatus for introducing organic molecules into the gas phase, permitting the generation of stable, reproducible HHG spectra. We then demonstrate the extraction of dynamical information from 2D HHG datasets, quantifying the nuclear autocorrelation function in benzene as well as the relative emission phase between contributing orbital channels in toluene and o-xylene.
Supervisor: Marangos, Jon ; Tisch, John Sponsor: European Commission
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral