Deuterium fractionation in ultracold collisions using trapped molecular ions

20世纪初发展起来的量子理论的关键要素之一是波粒二象性的概念。因此，粒子，如电子，可以以类似x射线的方式衍射，反过来，光可以被认为是由被称为光子的粒子状能量包组成的。在这个项目中，我们的目标是在非常接近绝对零度的温度下探索化学过程，在那里波粒二象性的概念变得非常重要。当物质冷却时，组成原子和分子的动能减少，运动更慢，因此它们的动量减小。在量子理论中，这意味着它们的波长增加，它们的波状特性增强。传统的化学过程“粒子观”将这些过程描述为分子之间的碰撞，打破并形成新的化学键。然而，在非常低的温度下，化学转化的基本物理描述发生了变化；经典碰撞的图像需要用波动的描述来代替，用隐含的叠加和干涉效应来代替。这个跨学科项目的具体目标是精确测量氨阳离子和甲基阳离子在气相中与H2O和NH3等中性物质的超冷反应。我们打算探索反应的固有波性质，如隧道，势垒反射和干涉如何在决定反应速率作为温度函数时变得重要。离子中性反应往往具有非常低的能垒，因此即使在最低温度下也可以非常快。低动能使得反应的行为对分子间的远程作用力高度敏感。超冷碰撞提供了利用电磁场进行新形式控制的可能性。因此，这是研究化学动力学的一个非常不寻常的机制，但也非常适合测试化学反应性的量子理论。在接近10 K的温度下，研究变得与理解星际气体丰富多样的化学成分相关。我们将研究中性分子中一个或两个氢原子被一个氘原子取代的反应。这种类型的反应在星际介质的背景下非常重要，因为它们决定了空间中氘化分子的浓度，这些可以与宇宙形成的基本理论联系起来。实验上，我们采用了新的装置来控制反应物质的温度和运动。利用射频场将离子捕获在真空室的高度局部区域，并利用激光冷却技术保持在非常低的温度。离子与Stark减速器提供的中性分子束相互作用，Stark减速器是一种可以使极性分子减速的装置。将其与离子阱相结合，为我们提供了一个独特的、国际领先的装置，以无与伦比的控制来测量超冷反应碰撞。为了测量反应速率，将开发新的技术来测量离子的数量和种类随时间的变化。这可以通过施加额外的电场来震动被捕获的离子来实现。离子的最终运动是由存在的反应物和生成物离子的数量及其质量决定的。新的基于激光的技术也将用于控制和监测分子的内部运动。在这个项目范围内开发的技术是新颖和及时的，毫无疑问，将在化学物理中发展最迅速的领域之一产生重大影响。反应速率的测量也将对天体物理学和计算化学产生潜在的影响。

One of the key elements of the quantum theory developed in the early 20th century is the concept of wave-particle duality. Thus particles, such as electrons, can be diffracted in a manner similar to X-rays, and conversely light can be considered to be composed of particle-like energy packets known as photons .In this project we aim to explore chemical processes at temperatures very close to absolute zero where these concepts of wave-particle duality become very important. When matter is cooled, the constituent atoms and molecules have less kinetic energy and move more slowly, hence their momentum is decreased. In the quantum theory this implies that their wavelength increases and their wave-like characteristics are enhanced. The conventional 'particle-view' of chemical processes describes these as occurring through collisions of molecules with one another, breaking and making new chemical bonds. At very low temperatures, however, the fundamental physical description of a chemical transformation changes; the picture of classical collisions needs to be replaced with a description in terms of wave motion, with the implied effects of superposition and interference. The specific goal of this interdisciplinary project is the precise measurement of ultracold reactions of ammonia cations and methyl cations in the gas phase with neutral species such as H2O and NH3. We intend to explore how the intrinsic wave properties of the reaction like tunneling, barrier reflection and interference become important in determining the rate of the reaction as a function of temperature. Ion-neutral reactions tend to have very low energy barriers and hence can be very fast even at the lowest temperatures. The low kinetic energy makes the behaviour of the reaction highly sensitive to the long-range forces between the molecules. Ultracold collisions offer the possibility of new forms of control using electromagnetic fields. This is therefore a very unusual regime for investigating chemical dynamics, but also one well suited for testing quantum theories of chemical reactivity. At temperatures near 10 K the studies become relevant to understand the rich and diverse chemistry of interstellar gas. We will study reactions in which one or two of the hydrogen atoms in the neutral molecule have been replaced with a deuterium atom. Reactions of this type are very important in the context of the interstellar medium because they determine the concentrations of deuterated molecules in space and these can be related back to fundamental theories of the formation of the universe.Experimentally we employ novel devices for controlling the temperature and motion of the reacting species. The ions are trapped in a highly localized region of a vacuum chamber using radiofrequency fields and are maintained at very low temperatures using the technique of laser cooling. The ions interact with a beam of neutral molecules provided by a Stark decelerator , a device which can decelerate polar molecules. Combining this with an ion trap provides us with a unique and internationally leading set-up to measure ultracold reactive collisions with unparalleled control. In order to measure the rates of reactions, novel techniques will be developed to measure the change in the numbers and species of the ions over time. This can be done by applying additional electric fields which shake the trapped ions. The resultant motion of the ions is characteristic of the number of reactant and product ions present and their masses. New laser-based techniques will also be used to control and monitor the internal motions of the molecule. The techniques which will be developed in the scope of this project are novel and timely and will undoubtedly have high impact in one of the most rapidly developing fields of chemical physics. The reaction rate measurements will also have potential impact in Astrophysics as well as computational chemistry.