Electrons at the water/air interface

在其复杂的结构中，已知液态水支持可以容纳电子的空腔。这种电子被称为水合电子，由于其在化学、物理和生物学中的广泛重要性，已经被研究了几十年。目前的提议提出了一项可行性研究，以观察水中的多余电子，这些电子不局限于空腔内，而是驻留在水面上。这一提议的部分动机来自最近的预测和观察，即某些离子优先结合在水/空气界面的水表面。最近，首席研究员和同事们也观察到了电子与大型气相水团簇表面的结合。这一观察结果引发了一场关于这类系统中电子结合问题的实质性辩论。在目前的建议中，我们提出了一种将这两种意见结合在一起的方法。具体来说，我们试图研究束缚在无限团簇表面的电子，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，对大气化学来说，它在海水气溶胶颗粒表面呈现出一种潜在的高活性物质。在生物学中，表面结合的电子同样是低能电子的来源，众所周知，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）的相互作用已经被讨论了几十年。如前所述，某些离子有在水面结合的倾向。其中之一是碘离子阴离子，它在水/空气界面的浓度急剧增加。我们将用这个阴离子注入一个电子到水面上，利用它所谓的电荷转移到溶剂激发。通过用超短脉冲驱动这种转变，其持续时间比水分子重新排列所需的时间短，我们有效地将电子注入到水面上。表面的水分子与负电荷发生强烈的相互作用，然后重新组织以容纳电子。因此，电子最初将被束缚在水面上，在那里我们将能够探测到它。我们的探测依赖于表面对强入射辐射的微弱响应。这是基于这样一个事实，即在一个界面上，反转对称性必然被打破，这导致光子以两倍于入射辐射频率发射，并被称为二次谐波产生（SHG）。因为这个过程只发生在界面上，所以它是高度表面特异性的。如果驱动场或SHG辐射与表面的一种跃迁发生共振，则该过程可以大大增强，我们将使用表面电子的第一电子跃迁。由于在这个能量下，表面上没有其他任何东西发生共振，因此单由于电子的存在，SHG将被增强。在这种情况下，SHG也具有物种选择性。在实验上，我们已经在一个明确的时间内创造了电子，现在可以使用第二个超短探针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，通过调整探针波长来表征它。此外，我们还可以通过在产生脉冲和探测脉冲之间引入延迟来监测电子随着时间的推移变得更加溶剂化时的弛豫动力学。通过这种方式，我们可以深入了解这些外来电子的超快弛豫动力学。由于这是一项可行性研究，该项目的完成将激发一些研究轨道，旨在更详细地了解溶剂化动力学并调查其反应性。

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.