Imaging Chemical Dynamics with Ultrafast Laser Spectroscopy

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This programme reveals how molecules change on the femtosecond timescale and is motivated by the principle that chemical function depends on form. Observing structural dynamics during chemical reactions reveals information that can infer molecular behaviour or be used to synthesise new pharmaceuticals and catalysts. Such measurements conventionally use spectroscopy, which characterizes molecules according to the light they absorb at different frequencies. These 'fingerprint' spectra are observed over time to follow a reaction. However, molecules do not necessarily absorb light at easily accessible frequencies, meaning that important chemistry is often spectroscopically dark.The emergence of a new technique in physics, Coulomb explosion imaging, opens the exciting possibility of investigating this dark chemistry directly. This method uses an intense and ultrafast laser pulse (35 fs) to quickly remove binding electrons from a molecule, leaving many positively charged sites that explode into fragments due to Coulomb's law. Measuring and correlating the relative velocities of these fragments as a function of time allows the shape of the molecule before the explosion to be reconstructed at different stages of a reaction. This research will initiate photochemical reactions and probe their structural changes using Coulomb explosion imaging. Its key aim will be to observe complete reactions, particularly as they pass through short-lived structures, including intermediates and transition states that are of fundamental importance to controlling the reactivity of a molecule, and for predicting such behaviour computationally. This programme will also be the first to directly image spectroscopically dark biological photochemistry, and could reveal information on vital reactions, such as the stability of DNA with respect to UV light. Revealing this unknown chemistry will enable greater control of these mechanisms, leading to new light-driven chemistry or devices in the life and physical sciences.The milestones outlined above will be reached through three projects. The first will develop a Coulomb explosion imaging experiment at the University of Oxford for the analysis of biomolecules isolated through electrospray ionisation. This will create a unique pathway to image structural biology that does not require crystallography, and which will be used as a starting point for investigating biomolecular dynamics. These will be investigated through the remaining two projects. One will develop a purpose-built tabletop instrument at Oxford to record 'molecular movies' of fundamental chemistry using time-resolved Coulomb explosion imaging. The final project will be undertaken using the FLASH free electron laser at the Deutsches Elektronen Synchrotron (DESY), which allows molecular structures to be site-selectively ionised. My group will collaborate with researchers at DESY to use this selectivity to study charge transport in nucleobases and aromatic amino acids, revealing new insights into their essential chemistry.