A New Effect in Ultrafast X-ray Scattering

光会引发许多重要的化学反应。这些技术包括光合作用(将阳光转化为化学能)、人类视觉(通过光诱导的分子变化来探测光子)，以及新技术，如癌症的光动力疗法、光催化、用于医疗诊断的荧光标签和光伏技术。分子中的光触发过程很难进行实验研究，而且涉及分子结构的协调变化和分子中电子的快速重排的复杂相互作用。锥形交叉点对光化学反应的结果起着决定性的作用，类似于标准基态化学中的过渡态。这些是光化学路径上的区域，分子可以在这些区域有效地在电子态之间转换。能够通过锥形交叉点绘制分子的路径图将为通过修改激发态动力学来控制光化学反应开辟道路。要做到这一点，我们必须同时观察分子的电子特性和相应的分子结构变化。这一挑战因所涉及的时间尺度较短而变得更加复杂，大约为飞秒级。值得注意的是，一秒钟的飞秒和3000万年中的几秒一样多。相比之下，标准的结构确定技术需要很长的观测时间。被称为X射线自由电子激光(XFELs)的新设备可以发射极短脉冲的高强度高能X射线光子，使全新类型的测量成为可能。在最近的工作中，我们证明了我们可以跟踪受激发分子中分子结构的变化，并在单独的实验中检测到当分子吸收光时电子几乎瞬间的重新排列。利用这些进展，拟议的项目将开发跟踪电子运动和原子核运动的测量方法，从而能够识别圆锥形交叉点，并确定圆锥形交叉点的分子结构。由此产生的实验技术将为基础研究提供一个强大的工具，并提供可用于定制光活性分子的电子和原子核的图像，最终为催化、新的癌症治疗和从阳光中获取能量的新技术做出贡献。

Light triggers many important chemical reactions. These include photosynthesis (converting sunlight to chemical energy), human vision (detecting photons via light-induced changes in molecules), and new technologies such as photodynamic therapy for cancer, photocatalysis, fluorescent tags for healthcare diagnostics, and photovoltaics. Light-triggered processes in molecules are difficult to study experimentally and involve a complex interplay of concerted changes in molecular structure and rapid rearrangements of the electrons in the molecule. Conical intersections play a decisive role for the outcome of photochemical reactions, analogous to that of a transition state in standard ground-state chemistry. These are regions on photochemical pathways where molecules can transition efficiently between electronic states. Being able to map the path of molecules through conical intersections would open avenues to controlling photochemical reactivity via modification of excited state dynamics. To achieve this we must simultaneously observe the electronic characteristics of the molecule and the corresponding changes in molecular structure. The challenge is compounded by the short timescales involved, on the order of femtoseconds. Notably, there are as many femtoseconds in a second as there are seconds in 30 million years. In contrast, standard techniques for structural determination require long observation times. New facilities known as X-ray Free-Electron Lasers (XFELs) deliver extremely short pulses of intense high-energy x-ray photons, making completely new types of measurements possible. In recent work, we have demonstrated that we can track the changes in molecular structure in excited molecules and, in separate experiments, detect the nearly instantaneous re-arrangement of electrons when molecules absorb light. Exploiting these advances, the proposed project will develop measurements that track the motion of electrons alongside the motion of the nuclei, allowing conical intersections to be identified, and the structure of molecules at conical intersections to be determined. The resulting experimental technique will yield a powerful tool for fundamental research and provide images of electrons and nuclei that can be used to customise photoactive molecules, ultimately contributing to new technologies in catalysis, new cancer treatments, and energy harvesting from sunlight.