Low temperature ion-radical collisions

各种气体环境的化学反应主要涉及瞬变的、高活性的原子和分子物种；这些环境包括星际介质(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、电放电和等离子体。存在的关键的高活性物种要么是自由基--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‘不成对的’--要么是带有正电荷(阳离子)的离子物种。这类物种有形成新化学键的自然倾向，通常这些物种的反应具有非常低的激活能，甚至是零激活能。这意味着它们通常即使在低温下也会有非常快的反应--事实上，随着温度的降低，这些物种的许多反应会变得更快。为了对上述环境(可能包含数百种不同的化学物种)的化学成分进行建模，我们需要知道这些物种的反应速度有多快，以及这些反应速率如何随温度变化。对于星际介质，温度很低(10-50开尔文)，而在火焰和等离子体中，温度可能很高。因此，已知室温下的反应速率通常不能为建模提供足够的信息。在这项工作中，我们将在较宽的温度范围内(从300Kelvin以上到1Kelvin以下)测量自由基物种和离子物种之间的反应速率。目前在测量两个暂态物种之间的反应速率方面存在着巨大的知识差距--大多数工作都是在一个暂态物种和一个稳定物种上完成的。目前缺乏实验室测量信息的原因是，参与反应的两种瞬时物种的浓度往往很低，因此目前的方法缺乏检测反应发生的灵敏度-每秒形成的反应产物分子的数量可能低到无法检测的程度。为了进行这些测量，我们将组装一种独特的仪器，它包括一个用于产生具有可变动能的自由基物种(因此也包括可变温度)的‘塞曼减速器’，以及一个用于产生用于反应的冷靶离子物种的激光冷却离子陷阱。塞曼减速器利用了这样一个事实，即自由基物种通常是有磁性的(因为它们有未配对的电子)，因此它们的速度可以通过磁场来控制--在这种情况下，磁场是由12-100个螺线管线圈组成的线性序列，自由基通过这些线圈。通过减速自由基物种(H、N和O原子，或CH3和CN分子)，我们可以控制它们的动能，从而控制温度。对于离子物种，我们使用射频四极杆来捕获离子(由中性前体激光电离产生)，并通过使用激光冷却，我们可以创建低密度的原子离子云(在这种情况下是Ca+)。这些离子凝聚成一种“库仑晶体”，离子在其中以规则的阵列排列，温度可以低至几毫克开尔文。钙离子不断地发出荧光，可以通过成像显微镜单独观察。分子离子(在这种情况下是CH+、C2H2+、Co2+或C6H6+)可以被共凝聚(“共冷”)到库仑晶体中，并在那里被捕获几个小时。在实验中，塞曼减速器中的自由基与被捕获的离子相互作用，发生反应产生一种新的离子化学物种，它也被捕获。因此，反应速度是通过监测产物离子形成随时间的变化来确定的。我们提议的实验的独特能力来自于离子非常长的捕获时间和甚至观察到陷阱中的单个离子的能力的结合。

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.