Isomerisation of gas-phase structures with Coulomb explosion imaging.

This project falls within the EPSRC 'Chemical Reaction Dynamics and Mechanisms' area, as well as contributing to 'Light Matter Interactions and Optical Phenomena' and 'Computational and Theoretical Chemistry'. This work aligns with the EPSRC 'Productive Nation' prosperity outcome. Coulomb explosion imaging was developed in the 1980s; by firing small molecules through thin solid films, it was shown that they could be stripped of their electrons, causing subsequent explosion into charged fragments due to electrostatic repulsion. Developments in laser technology lead to a resurgence of interest in Coulomb explosions in the 1990s since fundamental ionisation processes could be easily accessed through interactions with the electric field of the laser pulse. The current accessibility of x-ray free electron lasers (XFEL) has once again been followed by an increased interest in strong field-matter interactions. These modern techniques yield particularly interesting results with respect to chemical reaction dynamics as such pulse durations are on the same timescale as molecular dynamics. The tunability of these instruments allow control of many reaction parameters, such as selectively promoting nuclear and electronic wave-packets to desired states and potential surfaces, and manipulating reaction time dependencies. Pump-probe procedures allow the operator to excite (and propagate), and Coulomb explode a molecular system with two respective laser pulses with energies selected to access specific reaction mechanisms. The time between pulses can be varied to visualise the time evolution of a reaction mechanism. This project will apply ultrafast table-top and XFEL techniques to understand the chemical reaction dynamics of gas phase isomers, improve upon mass-spectrometry techniques, and add to the fundamental understanding of complex and convoluted light-matter interactions. Current mass spectrometry methods are limited by their inability to differentiate between structural isomers and, as a result, allow little to be inferred about the stages of a chemical evolution. By following isomerisation reactions with pump-probe and delay techniques, significant changes in molecular structure can be resolved as a function of time, allowing an enriched understanding of the way molecules transition between structural isomers.
This work will focus on laser-induced interhalogen elimination of di-halogenated methane (CH2X2, where X represents a halogen), which sees the formation of a bond between the two halogen substituents which are then ejected as a diatomic halogen species. This process is expected in the liquid phase of such methane derivatives, and theoretically expected for the gas-phase. Experimental evidence confirming or correcting these predictions is novel. Ultrafast techniques will be used to follow the bond formation and breaking mechanisms and to separate and identify the gas-phase isomers of di-halogenated methane. The characteristics of the fragments produced will be used to explain the underlying intramolecular halogen interactions in these structures. Further work will form a case study on the CF2X2 series which-computationally-are predicted to undergo interhalogen elimination more readily than their CH2X2 counterparts. The use of both tabletop and XFEL techniques, allowing selective access to different electronic states of a molecule, will not only allow the differentiation of structural isomers, but also add to the understanding of ground and excited state contributions. By visualising internuclear separation as a function of time, inferences may be made on the complex relationships between potential energy surfaces and electronic and nuclear population, and theoretical understanding may be improved upon through collected empirical and experimental data. Separating and confirming the presence of structural isomers is an important aim for organic and biochemical synthesis.