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Low temperature ion-radical collisions

低温离子自由基碰撞

基本信息

批准号：
EP/N004647/1
负责人：
Tim Softley
金额：
$ 76.43万
依托单位：
University of Birmingham
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2016
资助国家：
英国
起止时间：
2016 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FN004647%2F1
关键词：
Low temperature ion radical collisions

项目摘要

The chemistry of various gaseous environments is dominated by reactions involving transient, highly-reactive atomic and molecular species; these environments include the interstellar medium (ISM, principally very low density gas clouds between the stars) the upper atmosphere, flames and combustion systems, electric discharges and plasmas. The key highly-reactive species present are either free-radicals - atoms or molecules which generally have an odd number of electrons so that at least one electron is 'unpaired' - or ionic species carrying a positive electrical charge (cations). Such species have a natural tendency to form new chemical bonds, and typically reactions of these species have very low activation energies, or even zero activation energy. This means that they typically have very fast reactions even at low temperatures - indeed many reactions of these species become faster as the temperature lowers. In order to model the chemistry of the environments above (which may contain hundreds of different chemical species), we need to know how fast the reactions of these species are, and how those reaction rates vary with temperature. For the interstellar medium the temperatures are very low (10 - 50 Kelvin) whereas in flames and plasmas the temperature may be very high. Thus knowing the reaction rates at room temperature does not generally provide sufficient information for modelling purposes.In this work we will measure the rates of reactions between free-radical species and ionic species over wide temperature ranges (from above 300 Kelvin to below 1 Kelvin). There is currently a vast knowledge gap in terms of measuring rates for reactions between two transient species - most work has been done with one transient species and one stable species. The reason for the current dearth of information from laboratory measurements is that both of the transient species involved in the reaction tend to be present in very low concentrations, and therefore current methods lack the sensitivity to detect the occurrence of reactions - the number of reaction product molecules formed per second is likely to be undetectably low.To make these measurements we will assemble a unique instrument which consists of a 'Zeeman decelerator' for producing the free-radical species with variable kinetic energies (and hence variable temperatures), and a laser-cooled ion trap for producing the cold target ionic species for reaction. The Zeeman decelerator uses the fact that free-radical species are typically magnetic (as they have unpaired electrons) and so their velocities can be controlled using magnetic fields - in this case the fields are created by a linear sequence of 12-100 solenoid coils through which the radicals pass. By decelerating the radical species (H, N and O atoms, or CH3 and CN molecules) we can control their kinetic energy and hence the temperature. For the ionic species we use a radiofrequency quadrupole to trap the ions (which are produced by laser ionization of neutral precursors), and by using laser cooling we can create a low density cloud of atomic ions (Ca+ in this case). The ions condense to form a 'Coulomb crystal' in which the ions take up positions in a regular array and the temperature can be as low as a few milli-Kelvin The Ca+ ions are constantly fluorescing and can be observed individually by imaging microscopy. Molecular ions (in this case CH+, C2H2+, CO2+, or C6H6+) can then be co-condensed ('sympathetically cooled') into the Coulomb crystal and trapped there for periods of hours.In the experiments, radicals from the Zeeman decelerator interact with trapped ions, and reactions occur producing a new ionic chemical species, which is also trapped. Thus the reaction rate is determined by monitoring product ion formation versus time. The unique ability of our proposed experiment derives from a combination of the very long trapping time of the ions and the capability to observe even single ions in the trap.

各种气体环境的化学反应主要是涉及瞬态，高活性原子和分子物种的反应;这些环境包括星际介质（ISM，主要是恒星之间的极低密度气体云），高层大气，火焰和燃烧系统，放电和等离子体。存在的关键高活性物质是自由基-通常具有奇数个电子的原子或分子，因此至少有一个电子是“未配对的”-或携带正电荷的离子物质（阳离子）。这样的物质具有形成新化学键的天然倾向，并且这些物质的反应通常具有非常低的活化能，或者甚至零活化能。这意味着它们通常即使在低温下也具有非常快的反应-事实上，这些物种的许多反应随着温度降低而变得更快。为了模拟上述环境的化学性质（可能包含数百种不同的化学物质），我们需要知道这些物质的反应速度有多快，以及这些反应速率如何随温度变化。对于星际介质，温度非常低（10 - 50开尔文），而在火焰和等离子体中，温度可能非常高。因此，知道在室温下的反应速率一般不提供足够的信息建模purposes.In这项工作中，我们将测量自由基物种和离子物种之间的反应速率在很宽的温度范围内（从高于300开尔文低于1开尔文）。目前在测量两种瞬态物种之间的反应速率方面存在巨大的知识差距-大多数工作都是用一种瞬态物种和一种稳定物种完成的。目前缺乏实验室测量的信息的原因是，参与反应的两种瞬态物质往往以非常低的浓度存在，因此目前的方法缺乏检测反应发生的灵敏度-每秒钟生成的反应产物分子的数量可能低得无法检测。为了进行这些测量，我们将组装一个独特的仪器，该仪器包括：“塞曼减速器”，用于产生具有可变动能（因此可变温度）的自由基物质，以及激光冷却离子阱，用于产生用于反应的冷目标离子物质。塞曼减速器利用自由基物质通常具有磁性（因为它们具有未成对电子）的事实，因此可以使用磁场控制它们的速度-在这种情况下，场由自由基通过的12-100个螺线管线圈的线性序列产生。通过使自由基物质（H、N和O原子，或CH 3和CN分子）减速，我们可以控制它们的动能，从而控制温度。对于离子种类，我们使用射频四极杆来捕获离子（由中性前体的激光电离产生），并且通过使用激光冷却，我们可以创建低密度的原子离子云（在这种情况下为Ca+）。离子凝聚形成“库仑晶体”，其中离子占据规则阵列的位置，温度可以低至几毫开尔文。Ca+离子不断发出荧光，可以通过成像显微镜单独观察。分子离子（在本例中为CH+、C2 H2+、CO2+或C6 H6+）可以共凝聚（“同冷”）到库仑晶体中，并在那里被捕获数小时。在实验中，来自塞曼减速器的自由基与被捕获的离子相互作用，发生反应产生新的离子化学物质，也被捕获。因此，通过监测产物离子形成与时间的关系来确定反应速率。我们所提出的实验的独特能力来自于离子的非常长的捕获时间和在阱中观察甚至单个离子的能力的组合。