The composition of the Earth’s atmosphere is complex and vast, yet our ability to understand the photochemical reactions of the species in the atmosphere is limited despite its importance in assessing the impact of man-made emissions on the environment. An advanced spectroscopic technique, time-resolved Coulomb explosion imaging (TR-CEI), or a camera for molecules moving extremely fast, would increase our understanding of the reactions in the atmosphere. This technique consists of using an intense laser to shoot at a molecule to remove a lot of electrons, or what’s holding the atoms together, resulting in something so unstable it has no other choices than “explode” into charged pieces. Following the Coulomb explosion, those charged pieces have a relative momentum, which is dependent on the structure.
Capucine, recent EPSRC summer intern in the group, shares some of the results from her project.
Whilst the apparatus is under construction in the newly started Green group, simulations of the Coulomb explosion for Brown Carbon (BrC) molecules were done. BrC, or ultraviolet (UV) light absorbing aerosols, are compounds produced by biomass burning and other man-made emissions. The simulations helped with determining how the experimental data would look like and how to best represent the data. The former is done by using Coulomb’s law and Newtonian mechanics while the latter is done by defining what’s called a Newton frame, or which atom or atoms in the molecule we’re looking at can give us the best photo?
This technique can be thought as taking a snapshots of a molecule whilst it undergoes ultrafast processes, which is done by changing the time delay between two laser shots, enabling us to better understand key photochemical reactions in the atmosphere. Taking 2-nitrophenol (2NP) as an example due to its widely studied photochemistry, CE simulations were performed from predicted structures from literature. Figure 1 shows the predicted photochemistry of 2NP, as suggested by literature and Figure 2 shows how the CES signal changes along the photochemical reaction of 2NP as shown in Figure 1. From Figure 2, each column represents the different structures as 2NP moves along the PES (Figure 1) and each row is a different way at looking at the molecule. From comparing the graphs, the nitro group is rotating around the C-N bond, and the hydroxyl hydrogen is migrating to the nitro group, leading to the formation of HONO, which will then be released into the atmosphere. This is what we’d expect to observe from the experiments.
Capucine Henin