Electrons at the water/air interface

在其复杂的结构中，液态水被认为支持可以容纳电子的空腔。这种电子-被称为水合电子-已经被研究了几十年，因为它在化学，物理学和生物学中具有广泛的重要性。目前的提案提出了一项可行性研究，以观察水中多余的电子，这些电子不局限于空腔内，而是驻留在水面上。该提议的部分动机是最近的预测和观察，即某些离子优先结合到水/空气界面处的水表面。主要研究者和同事最近也观察到电子与大的气相水簇表面的结合。这一观察结果引发了关于此类系统中电子结合问题的实质性辩论。在目前的提案中，我们提出了一种将这两种意见结合起来的方法。具体来说，我们试图调查电子绑定到一个无限的集群的表面，即在水/空气界面。表面束缚电子的存在可能具有重要的多学科意义。例如，对于大气化学来说，它在海水气溶胶颗粒的表面上呈现出潜在的高活性物质。在生物学中，表面束缚电子类似地提供了低能电子的来源，已知低能电子会导致DNA损伤。最后，从化学物理学的角度来看，这是最基本的阴离子，它与水（或任何溶剂）的相互作用已经讨论了几十年。如上所述，某些离子倾向于结合在水的表面。其中之一是碘阴离子，其在水/空气界面处显示出浓度的急剧增加。我们将使用这种阴离子，利用其所谓的电荷转移到溶剂激发，将电子注入到水表面。通过用超短脉冲驱动这种转变，其持续时间小于水分子重新排列所需的时间，我们有效地将电子注入到水的表面。表面的水分子与负电荷强烈相互作用，然后重组以容纳电子。因此，电子最初将被束缚在水的表面，我们将能够检测到它。我们的检测依赖于表面对强入射辐射的非常微弱的响应。它是基于这样的事实，即在界面处，反转对称性必然被打破，这导致以入射辐射频率的两倍发射的光子的产生，并且被称为二次谐波产生（SHG）。由于该过程仅发生在界面处，因此具有高度的表面特异性。如果驱动场或SHG辐射与表面上的物种跃迁发生共振，我们将使用表面电子的第一电子跃迁，则该过程可以大大增强。由于在该能量下表面上没有其他任何东西发生共振，因此SHG将仅由于电子的存在而增强。在这种情况下，SHG也是物种选择性的。在实验上，我们已经在一个明确的时间产生了电子，现在可以使用第二个超短探测脉冲检测电子并监测发射的SHG。一旦表面电子被识别，我们将通过测量其电子吸收光谱，通过调整探测波长来识别它。此外，我们还可以通过在创建脉冲和探测脉冲之间引入延迟来监测随着时间的推移电子变得更加溶剂化的弛豫动力学。通过这种方式，我们可以深入了解这些奇异电子的超快弛豫动力学。由于这是一项可行性研究，该项目的完成将引发一系列研究，旨在更详细地了解溶剂化动力学并研究其反应性。

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.