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.

各种气态环境的化学因素主要由涉及短暂性，高度反应性原子和分子物种的反应。这些环境包括星际培养基（ISM，恒星之间的密度非常低的密度气云），上层大气，火焰和燃烧系统，电气排放和等离子体。存在的主要高反应性物种是自由基的 - 原子或分子通常具有奇数的电子，因此至少一个电子是“不配对的” - 或带有阳性电荷（阳离子）的离子物种。这种物种具有形成新化学键的自然趋势，通常这些物种的反应具有非常低的激活能，甚至零活化能。这意味着即使在低温下，它们通常也具有非常快的反应 - 实际上，随着温度降低，这些物种的许多反应变得更快。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种），我们需要知道这些物种的反应的速度以及这些反应的速度如何随温度变化。对于星际培养基，温度非常低（10-50 kelvin），而在火焰和等离子体中，温度可能非常高。因此，知道室温下的反应速率通常不会为建模目的提供足够的信息。在这项工作中，我们将在宽温度范围内衡量自由基物种和离子物种之间的反应速率（从300 kelvin以上的开kelvin到1凯文以下）。目前，在测量两个瞬态物种之间的反应率方面存在巨大的知识差距 - 大多数工作都是对一个瞬态物种和一种稳定物种完成的。当前从实验室测量中缺乏信息的原因是，反应所涉及的两个瞬时物种往往存在很低的浓度，因此目前的方法缺乏检测反应发生的敏感性 - 每秒形成的反应产物分子的数量可能是不可发现的，因此可以使这些测量范围供应范围。可变动能（因此温度可变）和激光冷却离子陷阱，用于产生冷靶离子物种进行反应。 Zeeman减速器使用这样一个事实，即自由基物种通常是磁性的（因为它们具有未配对的电子），因此可以使用磁场来控制其速度 - 在这种情况下，该场是由12-100螺线管线圈通过的线性序列创建的，而该磁管线圈通过其通过，而自由基通过。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，我们使用辐射频化四极杆来捕获离子（通过中性前体的激光电离产生），通过使用激光冷却，我们可以创建原子离子的低密度云（在这种情况下为Ca+）。离子凝结形成一个“库仑晶体”，其中离子在常规阵列中占据位置，并且温度可以低至几毫克的Ca+离子不断荧光，并且可以通过成像显微镜单独观察。然后可以将分子离子（在这种情况下为CH+，C2H2+，CO2+或C6H6+）共同传感（“交感神经冷却”）到库仑晶体中并将其捕获数小时。在实验中，从Zeeman减速器中的自由基与捕获的ies reactions相互作用。因此，反应速率是通过监测产物离子形成与时间来确定的。我们提出的实验的独特能力源于离子非常长的诱捕时间和观察陷阱中的单个离子的能力的组合。