Novel Non-linear Optical-Fibre Sources for Time-resolved Molecular Dynamics: Towards the Next Generation of Ultrafast Spectroscopy

发展对分子与光的相互作用的详细了解是非常重要的。例如，这与视觉和光合作用的基本生物过程以及保护生物体免受紫外线（UV）损害的光阻途径（如DNA和黑色素等系统中所见）高度相关。了解光分子相互作用对许多其他物种也至关重要，包括光稳定剂、光致变色聚合物、捕光复合物、防晒剂、光动力治疗药物和与大气/星际光化学相关的分子。因此，推进实验技术，以改善这种系统的研究是势在必行的。特别是，更多地了解重新分配分子中多余吸收能量的基本机制--以及最终如何更好地利用它们--具有深远的意义。使用“超快”飞秒激光脉冲，其持续时间与分子运动的时间尺度相当，是研究光与物质相互作用的强大方法。使用“泵浦-探测”技术在真实的时间中跟踪多余的能量重新分配：泵浦吸收有效地启动整个过程的动态“时钟”，然后通过探测器在一系列精确控制的延迟时间处询问系统，绘制出弛豫路径。时间分辨光电子成像（TRPEI）是这种一般方法的一个非常强大的变体，产生高度微分能量和角度分辨的信息，提供深入了解潜在的电子物理学。TRPEI的一个关键要求是使用可调谐的紫外飞秒脉冲进行泵浦（激发）和探测（电离）。然而，在这一光谱区域的操作效率极低，这限制了许多研究的可行性和范围。一种迅速出现的新技术，用于提供大大提高的（100- 1000倍）紫外线产生效率的增益，利用空心光子晶体光纤（HC-PCF）。这些还提供了对短波长光谱区域（<200 nm）的访问，这是通过更传统的手段不容易实现的。该项目的主要目的是利用HC-PCF提供的优势，并对模型发色团基序（较大生物分子中的光吸收中心）中的过量能量再分配进行详细，系统的研究。所选择的基序都涉及提供UV光保护功能，并且TRPEI的高度差异性，由最先进的量子化学计算支持，将对介导这些过程的基本机制产生许多新的见解。我们的研究还将揭示分子结构、动力学和光化学功能之间相互作用的一般原理，这些原理广泛适用于更广泛的物种，包括那些可能被商业开发的物种。该项目汇集了四名在超快激光、非线性光学、分子动力学和尖端计算方面具有互补技能的研究人员。HC-PCF源将被集成到TRPEI装置中，创建一个独特的最先进的仪器。详细评估该设备将包括一个新的单波长泵浦探测（SWPP）计划，提供了一个扩展的“视图”沿着弛豫路径，并产生增强的动态信息的发展。这开辟了令人兴奋的新的研究途径，我们将利用这一点，使用SWPP-TRPEI在三个不同的分子基序中进行过量能量再分配的研究，为自然界中发现的发色团提供起始模型（如目标部分所述）。我们的工作代表了实现下一代低成本桌面光源用于超快光谱的重要一步，我们预计我们的研究结果的传播将对这一主要研究领域产生持久的影响。

Developing detailed understanding of molecular interactions with light is of great importance. This is highly relevant, for example, to the fundamental biological processes of vision and photosynthesis, and also in photoresistive pathways (as seen in systems such as DNA and the melanin pigments) that protect living organisms from damage by ultraviolet (UV) light. Understanding light-molecule interactions is also of critical relevance for many other species, including photostabilizers, photochromic polymers, light harvesting complexes, sunscreens, photodynamic therapy drugs and molecules relevant to atmospheric/interstellar photochemistry. Advancing experimental techniques to improve the study of such systems is therefore imperative. In particular, learning more about the fundamental mechanisms that redistribute excess absorbed energy in molecules - and ultimately how to better utilize them - is of profound interest.The use of "ultrafast" femtosecond laser pulses with temporal durations comparable to the timescales of molecular motion is a powerful method for studying light-matter interactions. Excess energy redistribution is followed in real time using "pump-probe" techniques: pump absorption effectively starts a dynamical "clock" on the overall process and the system is then interrogated at a series of precisely controlled delay times by the probe, mapping out the relaxation pathways. Time-resolved photoelectron imaging (TRPEI) is an extremely powerful variant of this general approach, yielding highly differential energy- and angle-resolved information offering deep insight into the underlying photophysics. A key requirement for TRPEI is the use of tuneable UV femtosecond pulses for both pump (excitation) and probe (ionization). Operating in this spectral region is, however, extremely inefficient and this places restrictions on the feasibility and scope of many studies. A rapidly emerging new technology for providing greatly improved (100-1000x) gains in UV generation efficiency makes use of hollow-core photonic crystal fibres (HC-PCFs). These also offer access to short-wavelength spectral regions (<200 nm) that are not easily realized via more conventional means. The key aim of this project is to harness the advantages afforded by HC-PCFs and undertake detailed, systematic studies of excess energy redistribution in model chromophore motifs (the light-absorbing centres in larger biomolecules). The selected motifs have all been implicated in providing UV photo-protective function and the highly-differential nature of TRPEI, supported by state-of-the-art quantum chemistry calculations, will yield much new insight into the fundamental mechanisms mediating such processes. Our study will also reveal principles relating more generally to the interplay between molecular structure, dynamics, and photochemical function that are broadly applicable to a far wider range of species - including those that may be exploited commercially.The project brings together four researchers with complementary skills in ultrafast lasers, non-linear optics, molecular dynamics and cutting-edge computation. HC-PCF sources will be integrated into a TRPEI set-up, creating a unique state-of-the-art instrument. Detailed evaluation the device will include development of a novel single-wavelength pump-probe (SWPP) scheme that provides an expanded "view" along relaxation pathways and yields enhanced dynamical information. This opens up exciting new avenues of investigation and we will take advantage of this in using SWPP-TRPEI to perform studies of excess energy redistribution in three distinct molecular motifs providing starting models for chromophores found in nature (as detailed in the Objectives section). Our work represents a major step forward in realizing a next generation of low-cost table-top light sources for ultrafast spectroscopy and we anticipate that the dissemination of our findings will have lasting impact on this major research field.