Coherent Control and Manipulation of Natural and Un-Natural Parity Contributions to Electron Impact Ionization from Laser-Excited Atoms.

自然和非自然宇称对激光激发原子电子碰撞电离的相干控制和操纵。

• 批准号: EP/P00671X/1
• 负责人: Andrew Murray
• 金额: $ 57.85万
• 依托单位: University of Manchester
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2017
• 资助国家: 英国
• 起止时间: 2017 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FP00671X%2F1
• 关键词: Coherent; Control; Manipulation; Natural; Un

Plasmas are ubiquitous in nature, with more than 99% of the universe being in this fourth state of matter. Plasmas can consist of a mixture of ions, electrons and neutral atoms or molecules, depending upon their temperature. At very high temperatures (e.g. at the centre of the sun) all atoms are ionized, so only ions and electrons are present in these regions. At lower temperatures (such as at the surface of the sun and in the stellar atmosphere, in fluorescent and neon lights, in interstellar space, in ion lasers or in the earths ionosphere) a plasma consists of a mixture of charged and neutral particles. The neutral particles are atoms or molecules that are either in their ground state, or they may be in an excited state. Electrons and ions in the plasma can then collide with these neutral particles, leading to an exchange of energy resulting in further ionization or excitation. The most common collisions are with electrons, since they move most easily within the plasma. The interactions are complex in nature, and lead to many of the features observed from a plasma, such as the production of light in neon tubes and fluorescent lights, in lightning and in auroras. Physicists need to understand the dynamics of these collisions to allow them to accurately describe the plasma. This is important in a wide range of areas, from optimising the energy of plasmas used in science and industry, through to understanding and predicting the solar wind that affects our climate. It is hence important to provide accurate models of the collisions that can occur. In the research proposed here we will experimentally and theoretically study ionizing collisions with excited atoms for the first time. This will be a combined international effort, with experiments being conducted in Manchester, and theory being developed in the USA. Understanding collisions with excited atoms is important, as the collision probability is usually greater than for ground-state atoms. This is because their cross-section (which effectively defines their 'area') is often much larger than when they are in the ground state. This occurs since the excited electron is in a higher orbital, and so is effectively at a larger distance from the nucleus. Even if the density of excited atoms in the plasma is lower than for ground-state targets, they may hence be of equal or greater importance when describing the dynamics.Little is known about collisions with excited targets, since it is very difficult to produce a high density of excited atoms in the laboratory. This has recently changed, since we now have tuneable lasers that can create excited atoms in sufficient quantities to carry out the experiments. The University of Manchester has invested in a suite of lasers that allow these difficult experiments to be performed. To accumulate data of sufficient accuracy the laser wavelength has to be controlled to better than 1 part in 1 billion for long periods of time, which is extremely challenging. We have demonstrated that this is possible in a set of pioneering experiments, and as part of this new work we will build control systems to allow this precision and stability to be achieved for periods of up to several weeks. A significant advantage of exciting atoms using lasers is that we can 'shape' them prior to the collision occurring. We have discovered that this allows us to probe the collision in a unique way, so that different contributions to quantum models of the interaction can be rigorously tested. In particular, we found that so-called 'un-natural parity' terms are very important, as they can contribute up to 50% of the cross-section. These terms are not included in current plasma models, so we believe they are badly underestimating the effect of excited atoms within the plasma. The experiments proposed here provide a unique way to study these terms, and they will allow new and precise models to be developed as part of this international collaboration.

等离子体在自然界中无处不在，超过99%的宇宙处于物质的第四种状态。等离子体可以由离子、电子和中性原子或分子的混合物组成，这取决于它们的温度。在非常高的温度下（例如在太阳的中心），所有的原子都被电离，所以只有离子和电子存在于这些区域。在较低的温度下（例如在太阳表面和恒星大气中，在荧光灯和霓虹灯中，在星际空间中，在离子激光器中或在地球的电离层中），等离子体由带电粒子和中性粒子的混合物组成。中性粒子是处于基态或激发态的原子或分子。等离子体中的电子和离子然后可以与这些中性粒子碰撞，导致能量交换，从而导致进一步的电离或激发。最常见的碰撞是与电子的碰撞，因为它们在等离子体中最容易移动。这种相互作用在本质上是复杂的，并导致从等离子体中观察到的许多特征，例如在霓虹灯和荧光灯中产生光，在闪电和极光中产生光。物理学家需要了解这些碰撞的动力学，以便准确地描述等离子体。这在广泛的领域都很重要，从优化科学和工业中使用的等离子体能量，到理解和预测影响我们气候的太阳风。因此，必须提供可能发生的碰撞的准确模型。在本文的研究中，我们将首次从实验和理论上研究激发态原子的电离碰撞。这将是一项国际合作的努力，实验在曼彻斯特进行，理论在美国发展。理解与激发态原子的碰撞是很重要的，因为碰撞概率通常大于基态原子。这是因为它们的横截面（有效地定义了它们的“面积”）通常比它们处于基态时大得多。这是因为激发的电子处于更高的轨道，因此有效地与原子核相距更远。即使等离子体中激发态原子的密度低于基态靶，它们在描述动力学时也可能具有同等或更大的重要性。由于在实验室中很难产生高密度的激发态原子，因此对与激发态靶的碰撞知之甚少。这种情况最近有所改变，因为我们现在有了可调谐的激光器，可以产生足够数量的激发原子来进行实验。曼彻斯特大学已经投资了一套激光器，可以进行这些困难的实验。为了积累足够精确的数据，激光波长必须长时间控制在十亿分之一以上，这是非常具有挑战性的。我们已经在一系列开创性的实验中证明了这是可能的，作为这项新工作的一部分，我们将建立控制系统，以实现长达数周的精度和稳定性。使用激光激发原子的一个显著优点是，我们可以在碰撞发生之前“塑造”它们。我们发现，这使我们能够以一种独特的方式探测碰撞，从而可以严格测试对相互作用量子模型的不同贡献。特别是，我们发现所谓的“非自然平价”项非常重要，因为它们可以贡献高达50%的横截面。目前的等离子体模型中没有包含这些项，因此我们认为它们严重低估了等离子体中激发原子的影响。这里提出的实验为研究这些术语提供了一种独特的方法，它们将允许开发新的精确模型，作为这种国际合作的一部分。

项目成果

Triple differential cross-section measurements for electron-impact ionization of methane from a coplanar geometry to the perpendicular plane.

从共面几何形状到垂直平面的甲烷电子轰击电离的三重微分截面测量。

• DOI: 10.1063/1.5127121
• 发表时间: 2019
• 期刊: The Journal of chemical physics
• 作者: Harvey M

The AC-MOT Cold Atom Electron Source (CAES)

AC-MOT 冷原子电子源 (CAES)

• DOI: 10.1088/1742-6596/875/6/052020
• 发表时间: 2017
• 期刊: Conference Series
• 作者: Jones M

An undergraduate laboratory experiment to build and characterize a thermionic triode for use as an audio amplifier

本科生实验室实验，用于构建和表征用作音频放大器的热电子三极管

• DOI: 10.1088/1361-6404/aba997
• 发表时间: 2020
• 期刊: European Journal of Physics
• 影响因子: 0.7
• 作者: Murray A

An undergraduate laboratory experiment to determine the critical point of SF 6 using light scattering at selected wavelengths

使用选定波长的光散射确定 SF 6 临界点的本科生实验室实验

• DOI: 10.1088/1361-6404/aba225
• 发表时间: 2020
• 期刊: European Journal of Physics
• 影响因子: 0.7
• 作者: Murray A

Studying Cold Potassium Rydberg Atoms with an AC-MOT

使用 AC-MOT 研究冷钾里德伯原子

• DOI: 10.1088/1742-6596/875/3/022046
• 发表时间: 2017
• 期刊: Conference Series
• 作者: Harvey M