Research

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Gas-phase Photochemistry

The Green group wants to advance our understanding of photochemistry - chemistry initiated by the absorption of light. When a molecule absorbs light in the visible or ultraviolet, a molecule is typically driven to an excited electronic state. Following this excitation, the molecule can exhibit rich and varied chemistry, as is important in the atmosphere, in biology, in chemical synthesis, and even in space. Understanding how a molecule will behave following photoexcitation is extremely challenging however, with the relevant dynamics being governed by quantum mechanics and evolving over extremely short time (~femtoseconds, 10-15 s)  and length scales (~angstroms, 10-10 m).  

The Green group develops and applies state-of-the-art experimental techniques to image these ultrafast dynamics occurring in isolated gas-phase molecules.  

Example Photochemical Reaction

Coulomb Explosion Imaging

Coulomb explosion imaging relies on the rapid stripping of multiple electrons from a molecule. The polycation produced then violently breaks apart into small positively charged fragments. The relative direction of recoil of these fragment ions is sensitive to the initial structure of the molecule. Therefore, by measuring relative ion momenta we record a snapshot of the molecule prior to explosion. If we couple this with an ultrashort laser pulse, this enables a structural imaging tool on the femtosecond timescale.  

Example relative 3D ion momenta recorded following a Coulomb explosion. The distinct ‘islands’ of signal each arise from a specific hydrogen atom within the molecule.

Diffractive Imaging

If you shine a high-energy beam of light (X-rays) or electrons on a molecule, and record the scattering pattern, you will observe interference between scattering from different atomic sites within the molecule. This is similar to the famous Young’s double slit experiment, but on the molecular level. From subtle oscillations in the recorded scattering pattern, we can retrieve the pair distances between atoms in a molecule. As we now have access to femtosecond pulses of X-rays and high-energy electrons, we can use these techniques to track evolving nuclear structure on ultrafast timescales. 

X-ray Spectroscopy

Time-resolved X-ray spectroscopy is another way of following photoinduced dynamics that has been revolutionised by advancements in free-electron laser technology. While the techniques discussed above are mainly sensitive to nuclear dynamics, X-ray spectroscopies are more sensitive to electronic dynamics. By measuring the transition or ionization energies of specific core sites within a molecule, we can track, with atom-site specificity, how a molecule’s electronic structure evolves during a photoreaction.  

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