Trapping Ion-Molecule Reaction Intermediates

离子和分子之间的气相反应主导着高层大气、燃烧系统和星际介质等环境的化学。由于带正电的离子物种(阳离子)具有很高的活性，许多离子-分子反应是无障碍的，这意味着它们没有活化能。然而，这些反应过程远非简单；虽然反应可能没有能量障碍，但反应轨迹通常形成范德华中间体，必须克服水下障碍才能形成产物。此外，离子-分子反应通常表现出非阿累尼乌斯行为：其反应速率常数随着温度的降低而增加。因此，离子-分子反应在高层大气和星际介质等低温环境中发挥着越来越重要的作用。然而，在没有溶剂或环境影响的情况下，研究离子-分子反应中间体的实验方法非常少--特别是当这些中间体是阳离子的时候。因此，离子-分子反应机制在很大程度上仍然无法在低温下得到解释。在这项工作中，我们将利用寒冷、可控环境的诸多好处，引入一种新的分析仪器来探测反应中间体。在实验上，我将构建一种独特的仪器，包括一个低温冷却离子陷阱和一个集成质谱仪。一团钙离子将被保存在射频四极离子陷阱中。在激光冷却后，这些钙离子将凝聚成一种规则的结构，称为“库仑晶体”。由于激光冷却的钙离子不断发出荧光，我们可以用CCD摄像机直接观察它们在库仑晶体中的晶格位置。通过与激光冷却的离子进行有效的动能交换，其他非激光冷却的物质可以“交感地”冷却到晶体中。低温条件将确保保持对称冷却分子离子的初始量子态分布。预冷的反应物分子将通过泄漏阀或脉冲阀进入。我将稳定范德华反应中间体，使它们没有足够的能量来越过形成产物的障碍。这将通过与低温氦缓冲气体的碰撞来实现。我们可以通过各种互补的检测方法来表征物种和监测离子-分子反应，包括：荧光离子的实时成像、飞行时间质谱学、共振增强的多光子电离和共振增强的多光子解离。通过这种方式，我们可以在首次提出离子-分子捕获理论几十年后，在低温(T&lt；20K)下提供第一个严格的实验验证。俘获理论目前被纳入发生在星际介质和高层大气中的化学的重要模型中--其中承认“在冷核普遍存在的低温下研究的过程所占的比例极小。此外，对于那些可能产生不同系列产物的反应，这些不同通道的分支比率往往无法测量”[空间科学。使用这里提出的新的分析仪器，我可以测量这些基本的重要反应过程的速率--第一次阐明反应中间体和潜势垒对反应机理的影响。

Gas-phase reactions between ions and molecules dominate the chemistry of environments such as the upper atmosphere, combustion systems and the interstellar medium. As positively charged ionic species (cations) are highly reactive, many ion-molecule reactions are "barrierless", meaning that they have no activation energy. However, these reaction processes are far from simple; while there may be no energetic barrier to reaction, reactive trajectories typically form van der Waals intermediates and must overcome submerged barriers to form products. Additionally, ion-molecule reactions often display non-Arrhenius behaviour: their reaction rate constants increase with decreasing temperature. Thus ion-molecule reactions play an increasingly important role in low-temperature environments, such as the upper atmosphere and the interstellar medium. There are, however, remarkably few experimental methods for studying ion-molecule reaction intermediates in the absence of solvent or environmental effects - especially when these intermediates are cationic. As a result, ion-molecule reaction mechanisms are still largely unexplained at low temperatures. In this work, we will exploit the numerous benefits of cold, controlled environments to introduce a new analytical instrument for probing reaction intermediates.Experimentally, I will construct a unique apparatus comprising a cryogenically-cooled ion trap and an integrated mass spectrometer. A cloud of Ca+ ions will be held in a radiofrequency quadrupole ion trap. Following laser cooling, these Ca+ ions will condense to form a regular structure termed a "Coulomb crystal". As the laser-cooled Ca+ ions are continually fluorescing, we can directly observe their lattice positions in the Coulomb crystal using a CCD camera. Other non-laser cooled species can be "sympathetically" cooled into the crystal through the efficient exchange of kinetic energy with laser-cooled ions. The cryogenic conditions will ensure that the initial quantum state distribution of sympathetically-cooled molecular ions is maintained. Pre-cooled reactant molecules will be admitted through a leak valve or pulsed valve.I will stabilise the van der Waals reaction intermediates so that they have insufficient energy to surmount the barrier to product formation. This will be achieved through collisions with cryogenic helium buffer gas. Species can be characterised and ion-molecule reactions monitored through a variety of complementary detection methods, including: real-time imaging of the fluorescing ions, time-of-flight mass spectrometry, resonance-enhanced multi-photon ionisation, and resonance-enhanced multi-photon dissociation.In this way, we can provide the first stringent experimental verification of ion-molecule capture theories at low temperatures (T < 20 K), decades after they were first proposed. Capture theories are currently incorporated into important models of the chemistry occurring in the interstellar medium and upper atmosphere - where it is acknowledged that "the fraction of the processes which have been studied at the low temperatures prevalent in cold cores is extremely small. In addition, for those reactions that may proceed to different sets of products, the branching ratios to these different channels are frequently unmeasured" [Space Sci. Rev. 156, p13 (2010)].With the new analytical apparatus proposed here, I can measure the rates of these fundamentally important reaction processes - elucidating the influence of reaction intermediates and submerged barriers on the reaction mechanism for the first time.