High energy and high intensity probes of chemical dynamics

Abstract

Gas phase dynamics measurements have been performed using a series of techniques aimed at extending the observation window available to conventional probes. The probes used through the thesis are based on high harmonic spectroscopy, multiphoton ionisation probe photoelectron spectroscopy, and extreme ultraviolet photoelectron spectroscopy.

For the high harmonic spectroscopy experiments we have developed a new experimental set-up in Southampton so that high harmonic spectroscopy experiments can be performed. The development of the system has included the design, building and commissioning of a flat field spectrometer to provide improved signal detection and a more suitable energy range for detection of high harmonics generated from molecules. The development of the experiment is presented in chapter 3. A pump probe study of the Raman excitation of CCl4 has been performed demonstrating that high harmonic spectroscopy is sensitive to molecular vibrations, this work is presented in chapter 4.

Photoelectron spectroscopy experiments have been performed on both ammonia and carbon disulphide, the results are presented in chapters 5 and 6. The ammonia study employs a multi photon ionisation probe. By probing the dynamics via the intermediate E0 Rydberg state we access a different set of vibrational levels in the ion and maintain overlap with the ion state for an extended period of time. Higher lying ion vibrational states have an average excited state lifetime of approximately 200 fs which closely matches the lifetime associated with the non-adiabatic dissociation process while the lower vibrational states have a much longer lifetime of over 300 fs which more closely matches the adiabatic dissociation process. The different trajectories taken by the two competing dissociation pathways at the region of the conical intersection show up different excitation and ionisation propensity allowing us to isolate features related to each process.

By probing carbon disulphide with an extreme ultra violet probe all reaction intermediates have been observed through to the multiple dissociation products. This demonstrates the experimental capabilities whilst using a probe beam generated from high harmonics, whilst this adds a level of complexity to the experiment it has provided a complete observation of the dissociation dynamics. Whilst carbon disulphide is a simple molecular system we have demonstrated that there are complex motions in the electronic and nuclear dynamics of this system that have not been previously observed.

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ORCID for Russell Minns: orcid.org/0000-0001-6775-2977

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