Multi-dimensional electron spectroscopy with photons

电子和中性分子之间的反应是许多科学技术分支的基础，包括辐射化学和生物学、星际化学、半导体工业中的等离子体蚀刻和水修复。从基础科学的角度来看，该反应是最简单的反应之一，是物理化学的基石。尽管具有广泛的重要性，但这种看似简单的反应的许多方面仍然未知，研究也很少。这些反应看似复杂：(1)因为电子结构的变化（决定了化学键）和分子中原子的运动之间的内在相互作用，(2)因为进入的电子从未真正结合（即能量上，它总是可以离开）。后一点意味着，不可避免地，这些反应涉及寿命很短的不稳定物种。要探测这种瞬态物质，就需要能够在飞秒量级的分子时间尺度上捕捉整个反应的电子和几何结构（1 fs = 0.000 000 000 000 001 s）。然而，没有实验方法可以直接做到这一点。电子-分子反应的最佳探测方法是基于电子能谱的方法。在过去的几年里，我们的小组已经开发了类似的方法，使用光激发而不是电子激发，有一些重要的优点，但也有一些有限的缺点。在本提案中，我们克服了局限性，开发了可以实时探测电子-分子反应中电子和几何变化的方法。新方法是基于这样一种想法，即多余的电子可以以一种无扰动的方式束缚在中性分子上，我们将利用这些电子作为分子内的电子源，在能量和时间上都是明确的。然后，我们将应用这些方法来探测电子与核碱基的反应，这代表了DNA生物辐射损伤的关键步骤，并且产物形成的细节和实时激发态的动力学仍然是未知的。

The reaction between an electron and a neutral molecules underpins many branches of science and technology including for example, radiation chemistry and biology, interstellar chemistry, plasma etching in the semiconductor industry, and water remediation. From a basic science perspective, the reaction presents one of the simplest reactions and is a corner stone of physical chemistry. Despite this broad importance, many aspects of this seemingly simple reaction remain unknown and poorly studied. The reactions are deceivingly complex: (i) because of the inherent interplay between the changes in electronic structure (which dictates chemical bonding) and the motion of atoms in molecules, and (ii) because the incoming electron is never actually bound (i.e. energetically, it can always leave). This latter point means that inevitably, the reactions involve unstable species that are very short lived. To probe such transient species requires one to be able to capture the electronic and geometric structure of the entire reaction on the molecular timescale which is on the order of femtoseconds (1 fs = 0.000 000 000 000 001 s). However, there are no experimental methods available to do this directly. The best methods to probe the electron-molecule reaction are based on electron spectroscopic methods. In the past few years, our group has developed analogous methods that use optical excitation rather than electron excitation, with some important benefits, but also some limiting disadvantages. In this proposal, we overcome the limitations and develop methods that can probe the electronic and geometric changes in an electron-molecule reaction in real-time. The new methods are based on the idea that an excess electron can be bound to neutral molecules in an non-perturbing manner and we will exploit these electrons as an intramolecular source of electrons that is well-defined in energy and time. We will then apply these methods to probe the reaction of an electron with nucleobases, which represents a key step in biological radiation damage of DNA and for which the details of product formation and dynamics of excited states in real-time remain uncharted.