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

等离子体在自然界中无处不在，99%以上的宇宙都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。在较低温度下（如在太阳表面和恒星大气中，在荧光灯和霓虹灯下，在星际空间中，在离子激光中或在地球电离层中），等离子体由带电粒子和中性粒子的混合物组成。中性粒子是原子或分子，它们要么处于基态，要么处于激发态。然后，等离子体中的电子和离子可以与这些中性粒子碰撞，导致能量交换，从而进一步电离或激发。最常见的碰撞是电子，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，并且导致了从等离子体中观察到的许多特征，例如霓虹灯和荧光灯中的光的产生，闪电和极光。物理学家需要了解这些碰撞的动力学，才能准确地描述等离子体。这在很多领域都很重要，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，提供可能发生的碰撞的精确模型是很重要的。在这里提出的研究中，我们将首次从实验和理论上研究与激发原子的电离碰撞。这将是一项国际合作，实验将在曼彻斯特进行，理论将在美国发展。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。这是因为被激发的电子在更高的轨道上，因此它离原子核的距离更大。即使等离子体中激发态原子的密度低于基态目标，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受激原子，所以人们对与受激目标的碰撞知之甚少。这种情况最近发生了变化，因为我们现在有了可调谐激光器，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，使这些困难的实验得以进行。为了积累足够精确的数据，激光波长必须长时间控制在10亿分之一以上，这是极具挑战性的。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，以便在长达几周的时间内实现这种精度和稳定性。用激光激发原子的一个显著优势是，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，从而可以严格测试相互作用的量子模型的不同贡献。特别是，我们发现所谓的“非自然宇称”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括这些项，因此我们认为它们严重低估了等离子体中受激原子的影响。这里提出的实验提供了一种独特的方法来研究这些术语，它们将允许开发新的和精确的模型，作为国际合作的一部分。

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.