Trapping Ion-Molecule Reaction Intermediates

离子和分子之间的气相反应主导着上层大气、燃烧系统和星际介质等环境的化学。由于带正电荷的离子物种（阳离子）具有高度反应性，因此许多离子-分子反应是“无势垒”的，这意味着它们没有活化能。然而，这些反应过程远非简单;虽然可能没有反应的能量障碍，但反应轨迹通常形成货车德瓦尔斯中间体，并且必须克服淹没的障碍以形成产物。此外，离子-分子反应通常表现出非阿耳忒弥斯行为：它们的反应速率常数随着温度的降低而增加。因此，离子-分子反应在低温环境中发挥着越来越重要的作用，例如高层大气和星际介质。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时。因此，离子-分子反应机制在低温下仍然很大程度上无法解释。在这项工作中，我们将利用冷的，受控的环境引入一个新的分析仪器探测反应intermediate.Experimentally，我将构建一个独特的装置，包括一个低温冷却的离子阱和集成的质谱仪。一团Ca+离子将被固定在射频四极离子阱中。在激光冷却之后，这些Ca+离子将凝聚形成被称为“库仑晶体”的规则结构。由于激光冷却的Ca+离子不断发出荧光，我们可以使用CCD相机直接观察它们在库仑晶体中的晶格位置。其他非激光冷却的物质可以通过与激光冷却的离子有效地交换动能而被“同情地”冷却到晶体中。低温条件将确保维持交感冷却的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，我将稳定货车德瓦耳斯反应中间体，使它们没有足够的能量克服产物形成的障碍。这将通过与低温氦缓冲气体的碰撞来实现。可通过多种互补检测方法对物质进行表征并监测离子-分子反应，包括：荧光离子的实时成像，飞行时间质谱，共振增强多光子电离和共振增强多光子解离。通过这种方式，我们可以在低温下首次提供离子分子捕获理论的严格实验验证（T < 20 K），在它们首次被提出的几十年后。捕获理论目前被纳入星际介质和高层大气中发生的化学反应的重要模型中--在这些模型中，人们承认“在冷核中普遍存在的低温下研究的过程所占比例非常小。此外，对于那些可能产生不同产物集的反应，这些不同通道的分支比通常无法测量”[Space Sci. Rev. 156，p13（2010）]。使用本文提出的新分析仪器，我可以测量这些根本上重要的反应过程的速率-首次阐明反应中间体和浸没屏障对反应机理的影响。

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.