Chemical Applications of Velocity and Spatial Imaging

离子成像在25年前首次被证明，已经对我们在许多气相系统中探索分子变化（化学的本质）的方式产生了重大影响。该技术与质谱（MS）有共同的特点。这两种方法都是先从目标物质中移除一个电子，产生离子，即带电分子或碎片，然后根据它们的质量进行“分类”。在传统的质谱中，感兴趣的物质是通过其离子产率与质量的谱来表征的。在大多数离子成像实验中，电子的去除是由短脉冲激光引起的；产生的离子然后被加速到一个时间和位置敏感的探测器。较重的离子移动速度较慢，因此人们可以通过确保探测器只在适当的时间“打开”来成像特定质量的离子。当实验多次重复时，在探测器上建立的离子撞击的空间模式在视觉上是直观的，并提供了产生被监测产物的反应的定量能量信息。然而，目前的离子成像探测器的读出时间太慢，无法对在同一激光照射下形成的不同质量的离子进行成像，而且在目前的离子成像方案中，许多种类的离子不容易电离。因此，对一个给定反应的所有产物进行成像是非常耗时的（往好了说），往坏了说，是不可能的。我们寻求解决这两个限制。该团队的两名成员已经展示了新的、更快的、时间和位置敏感的传感器，能够在一次拍摄实验中对多个质量进行成像。这种多质量成像能力将进一步开发，并在整个团队中使用和改进。我们还提出了新的多光子电离方案以及基于使用更短激光波长或能量选择电子的短持续时间脉冲的“通用”离子形成方法。以下首要的科学目标将并行进行，并尽早利用离子成像技术的上述进展：(i)我们将使用最新的离子成像方法来探索气相中的分子变化，重点关注（光）化学反应的关键家族：有机分子的加成、解离、环化和开环反应，以及金属配体和金属簇的相互作用。这些选择反映了这些反应在合成、催化等方面的重要性，它们对互补的高水平理论的适应性，以及我们在溶液中探索相同反应的能力（使用新的超快泵浦探测激光光谱设备）。确定通过精细的气相研究建立的反应机制和能量学在多大程度上可以告诉我们对凝聚态反应性的理解，这是化学科学中当前的“热点”问题，该团队是解决这一问题的理想人选。（ii）我们将开发和利用结合质量、结构和空间分辨率的新的多维分析方法。质谱通常显示许多可归因于碎片离子的峰，但它们形成的途径往往不清楚。成像质谱被认为是一种揭示不同途径形成给定片段离子的新方法；区分和表征这些途径可以提供新的见解，例如，肽结构。然而，更雄心勃勃的是，我们建议将多质量和空间地图成像与现有的激光解吸/电离方法相结合，以实现表面和表面样品的空间分辨成分分析。这种能力将为各种活动提供新的机会，如组织成像（例如，检测组织标本中的金属离子，与了解一些金属对金属髋关节植入物的失效有关），法医分析(例如。指纹、油墨、染料、花粉等的“化学”成像)和平行质谱采样（例如血液样本）。

Ion imaging, first demonstrated just 25 years ago, is already having a major impact on the way we explore molecular change (the very essence of chemistry) in many gas phase systems. The technique has features in common with mass spectrometry (MS). Both start by removing an electron from the target species, generating ions, i.e. charged molecules or fragments, which are then 'sorted' by their mass. In traditional MS, the species of interest is characterised by its spectrum of ion yield versus mass. Electron removal in most ion imaging experiments is induced by a short pulse of laser light; the resulting ions are then accelerated towards a time and position sensitive detector. Heavier ions travel more slowly, so one can image ions of just one particular mass by ensuring that the detector is only 'on' at the appropriate time. The spatial pattern of ion impacts that builds up on the detector when the experiment is repeated many times is visually intuitive, and provides quantitative energetic information about the reaction(s) that yields the monitored product. However, the read out time of current ion imaging detectors is too slow to allow imaging of ions with different mass formed in the same laser shot, and many species are not readily amenable to ionisation in current ion imaging schemes. Imaging all products from a given reaction is therefore time consuming (at best) and, at worst, impossible.We seek to solve both these limitations. Two of the team have already demonstrated new, much faster, time and position sensitive sensors capable of imaging multiple masses in a single shot experiment. This multimass imaging capability will be developed further and rolled-out for use and refinement across the team. We also propose new multiphoton ionization schemes as well as 'universal' ion formation methods based on use of shorter laser wavelengths or short duration pulses of energy selected electrons. The following over-arching scientific ambitions will proceed in parallel, and exploit the foregoing advances in ion imaging technology at the earliest possible opportunity:(i) We will use the latest ion imaging methods to explore molecular change in the gas phase, focusing on key families of (photo)chemical reactions: addition, dissociation, cyclisation and ring opening reactions of organic molecules, and metal-ligand and metal-cluster interactions. These choices reflect the importance of such reactions in synthesis, catalysis, etc., their amenability to complementary high level theory, and our ability to explore the same reactions in solution (using a new ultrafast pump-probe laser spectroscopy facility). Determining the extent to which the mechanisms and energetics of reactions established through exquisitely detailed gas phase studies can inform our understanding of reactivity in the condensed phase is a current 'hot' issue in chemical science, which the team is ideally placed to address.(ii) We will develop and exploit new multi-dimensional analytical methods with combined mass, structural and spatial resolution. Mass spectra usually show many peaks attributable to fragment ions, but the paths by which these are formed are often unclear. Imaging MS is proposed as a novel means of unravelling different routes to forming a given fragment ion; distinguishing and characterising such pathways can offer new insights into, for example, peptide structure. Yet more ambitious, we propose to combine multimass and spatial map imaging with existing laser desorption/ionisation methods to enable spatially resolved compositional analysis of surfaces and of samples on surfaces. Such a capability will offer new opportunities in diverse activities like tissue imaging (e.g. detection of metal ions within tissue specimens of relevance to understanding the failure of some metal-on-metal hip implants), forensic analysis (e.g. 'chemical' imaging of fingerprints, inks, dyes, pollens, etc) and parallel mass spectrometric sampling (e.g. of blood samples).