Electrons at the water/air interface

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Within its complex structure, liquid water is known to support cavities that can accommodate an electron. Such an electron - known as the hydrated electron - has been studied for many decades because of its wide ranging importance in chemistry, physics and biology. The current proposal presents a feasibility study to observe excess electrons in water that are not confined within a cavity, but instead reside on the surface of water. The proposal is in part motivated by recent predictions and observations that certain ions preferentially bind to the surface of water at the water/air interface. Binding of an electron to the surface of large gas-phase water clusters have also recently been observed by the principal investigator and coworkers. This observation has prompted a substantial debate concerning the issue of electron binding in such systems. In the current proposal, we suggest a means of bringing these two observations together. Specifically, we seek to investigate electrons bound to the surface of an infinite cluster, i.e. at the water/air interface. The existence of surface-bound electrons may have important multidisciplinary implications. To atmospheric chemistry, for example, it presents a potentially highly reactive species on the surface of sea-water aerosol particles. In biology, the surface bound electron similarly presents a source of low energy electrons, which are known to cause DNA damage. Finally, from a chemical physics perspective, this presents the most elementary anion and its interaction with water (or any solvent) has been topical for many decades.As mentioned, certain ions have a tendency to bind at the surface of water. One of these is the iodide anion, which shows a dramatic increase in concentration at the water/air interface. We will use this anion to inject an electron onto the water surface using its so-called charge-transfer-to-solvent excitation. By driving this transition with an ultrashort pulse, which has a duration that is less than the time required for water molecules to rearrange, we effectively inject the electron onto the surface of the water. The water molecules at the surface interact strongly with the negative charge and will then reorganise to accommodate the electron. Thus, the electron will initially be bound to the surface of the water, where we will be able to detect it. Our detection relies on a very weak response of the surface to strong incident radiation. It is based on the fact that, at an interface, the inversion symmetry is necessarily broken, which leads to the generation of photons emitted at twice the incident radiation frequency and is termed second harmonic generation (SHG). Because the process only occurs at the interface, it is highly surface specific. The process can be greatly enhanced if either the driving field or the SHG radiation is in resonance with a transition of a species at the surface and we will use the first electronic transition of the surface electron. As there is nothing else on the surface that is in resonance at this energy, SHG will be enhance solely due to the presence of an electron. In this case, SHG is thus also species selective. Experimentally then, we have created the electron at a well-defined time and can now detect the electron using a second ultrashort probe pulse and monitor the emitted SHG. Once the surface electron is identified, we will characterise it by measuring its electronic absorption spectrum, by tuning the probe wavelength. Furthermore, we can also monitor the relaxation dynamics as the electron becomes more solvated with time, by introducing a delay between the creation and probe pulses. In this manner we can glean great insight into the ultrafast relaxation dynamics of these exotic electrons. As this is a feasibility study, the completion of the project will instigate a number of research tracks, aimed at understanding the solvation dynamics in more detail and investigating its reactivity.