Fs-VUV Generation: Mapping the Reaction Co-ordinate in Photochemical Dynamics

深入了解分子与光的相互作用是非常重要的。例如，它与发生在生物学中的基本过程特别相关，例如视觉和光合作用，以及发生在DNA和黑色素沉着系统中的所谓“自我保护”机制，以保护身体免受紫外线（UV）光的潜在破坏性影响。此外，光分子相互作用的理解对许多其他类型的分子至关重要，包括光稳定剂、光致变色聚合物、分子开关、光收集复合物和靶向递送活性剂的药物（光动力疗法）。因此，发展完善的实验技术来加强对这类系统的研究是一项重要的挑战。使用“超快”飞秒（fs）激光脉冲，其持续时间可与分子运动的时间尺度相媲美（1 fs = 10^-15 s），是研究光-物质相互作用的有力方法。利用“泵-探针”技术可以实时跟踪吸收光后分子内的能量再分配：泵启动能量再分配过程（有效地启动一个动态“时钟”），然后系统可以通过探针在一系列精确控制的延迟时间内被询问-绘制出能量流的路径。然而，这种方法的一个关键限制是，在许多情况下，沿着这些路径的完整“视图”受到限制，模糊了关键信息。解决这个问题是这项工作的主要目标之一。拟议的研究计划汇集了一组在超快激光，非线性光学，分子光谱和动力学，超高真空科学和尖端计算方法方面具有独特互补技能和经验的研究人员。在初始阶段，我们将开发一种经济且紧凑的光源，该光源将在电磁波谱的真空紫外（VUV）区域产生飞秒光脉冲。这将扩展最近发展的实验方法。源输出（我们称之为fs-VUV）非常适合用作泵-探针实验中的探针步骤，因为它提供了促进许多分子中多余能量再分配的途径的高度扩展视图（与使用非vuv探针相比）。这将使我们对光分子相互作用的本质有了前所未有的深入了解。在fs-VUV源的成功开发和表征之后，它将被用于升级赫瑞瓦特大学现有的一个实验，该实验使用泵浦探针光电子成像技术来研究能量再分配的动力学。在该项目的下一阶段，我们将使用fs-VUV探针来研究尿酸中的能量再分配。这是皮肤中主要的紫外线吸收剂之一（可能作为天然的“防晒霜”），我们的实验结果，结合支持的理论工作，将产生与这一重要的生物分子系统有关的重要的新机制信息。在项目的最后阶段，我们将使用fs-VUV作为光电子成像实验的探针，研究硝基苯及其某些选定衍生物的能量再分配和分子断裂。这些都是开发改进的光动力治疗药物的重要测试系统。特别是，我们将研究导致光诱导一氧化氮（NO）释放的产品通道，一氧化氮对许多生理重要功能的调节和维持很重要。我们的工作将开始提高对一般机制原理的理解，这些原理可以增强NO的生产渠道，并且可以很容易地扩展到更大的、实际适用的系统。

Developing a detailed understanding of how molecules interact with light is of great importance. For example, it is particularly relevant to fundamental processes that take place in biology, such as vision and photosynthesis, as well as in so-called "self-protection" mechanisms that occur in both DNA and the melanin pigmentation system, serving to protect the body from the potentially damaging effects of ultraviolet (UV) light. Additionally, an understanding of light-molecule interactions is of critical relevance for many other classes of molecules, including photostabilizers, photochromic polymers, molecular switches, light harvesting complexes and drugs for the targeted delivery of active agents (photodynamic therapy). Developing refined experimental techniques to enhance the study of such systems is therefore an important challenge.The use of "ultrafast" femtosecond (fs) laser pulses with temporal durations comparable to the timescales of molecular motion (1 fs = 10^-15 s) is a powerful method for studying light-matter interactions. Energy redistribution within a molecule following the absorption of light may be followed in real time using "pump-probe" techniques: the pump initiates the energy redistribution process (effectively starting a dynamical "clock") and the system may then be interrogated at a series of precisely controlled delay times by the probe - mapping out the pathways for energy flow. However, a key limitation with this approach is that, in many instances, the full "view" along these pathways is restricted, obscuring critical information. Addressing this issue forms one of the main goals of this work. The proposed research programme brings together a team of investigators with a unique set of complementary skills and experience in ultrafast lasers, non-linear optics, molecular spectroscopy and dynamics, ultra-high vacuum science and cutting edge computational methods. In the initial phase, we will develop an economic and compact light source that will produce femtosecond light pulses across the vacuum-ultraviolet (VUV) region of the electromagnetic spectrum. This will expand on recently developed experimental methods. The source output (which we refer to as fs-VUV) is well suited for use as the probe step in pump-probe experiments as it provides a highly expanded view of the pathways that facilitate excess energy redistribution in many molecules (when compared to using non-VUV probes). This will yield previously unobtainable levels of insight into the nature of light-molecule interactions.Following the successful development and characterization of the fs-VUV source, it will then be used to upgrade an existing experiment at Heriot-Watt University that uses pump-probe photoelectron imaging as a technique to study the dynamics of energy redistribution. In the next phase of the project we will use the fs-VUV probe to investigate energy redistribution in urocanic acid. This is one of the primary UV absorbers present in the skin (possibly acting as a natural "sunscreen") and our experimental results, in conjunction with supporting theoretical work, will yield important new mechanistic information relating to this important biomolecular system.In the final phase of the project, we will use the fs-VUV as a probe in photoelectron imaging experiments that investigate energy redistribution and molecular fragmentation in nitrobenzene and some of its selected derivatives. These are important test systems for the development of improved drugs for photodynamic therapy. In particular, we will investigate the product channel leading to light-induced release of nitric oxide (NO), which is important for the regulation and maintenance of many physiologically vital functions. Our work will begin to develop improved understanding of the general mechanistic principles that enhance the NO production channel and should be readily scalable to larger, practically applicable systems.