Electron-Atom Scattering in the Presence of a 1.17eV Laser Field

1.17eV 激光场中的电子原子散射

• 批准号: 1607140
• 负责人: Nicholas L.S. Martin
• 金额: $ 37.82万
• 依托单位: University of Kentucky Research Foundation
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-08-15 至 2021-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1607140&HistoricalAwards=false
• 关键词: Electron; Atom; Scattering; Presence; 1.17eV

This project examines the so-called free-free transitions of electrons exposed to laser light. In a gas of neutral atoms, bound atomic electrons may readily emit or absorb photons (particles of light, or electromagnetic radiation). However, in a partially ionized gas (a plasma), in addition to neutral atoms, ions and unbound (free) electrons are present. Because of the laws of the conservation of energy and momentum, these free electrons can only absorb (or emit) a photon during a collision with an atom. This process is known as a free-free transition, i.e., the electron is free before the collision and free after it, but has gained or lost an amount of energy equal to that of one (or more) photons that are present. It has been shown that free-free processes have a significant effect on the properties of astrophysical plasmas, such as those in the stellar (and solar) atmosphere and interstellar space, and also on the plasmas encountered in fusion research (fusion reactors) and the lighting industry (florescent tubes). It is therefore important to have a detailed knowledge of free-free processes, and these may be studied in the laboratory by scattering a beam of electrons from a gas jet in the presence of a laser beam. All experiments to date, except one, found that free-free transitions are independent of which atom (or molecule) is used. Very recently, an experiment carried out in Japan found the first experimental evidence that the type of atom can affect the transition. The experiments were carried out with Xenon atoms for which the effects were observable, but very small. This project will carry out similar experiments in Potassium, where a simple theory predicts that the effects will be ten times larger. The project will also develop what is known as a multipass system, where the laser beam (which is fired 30 times a second to produce laser pulses) will be bounced back and forth between mirrors at least ten times, and will therefore enable experiments to be carried out in one tenth of the time. The system will use a special "optical door" which will allow the laser pulse to be "injected" into the space between the two mirrors, before it is trapped. The target independence of free-free transitions is a requirement of the simple semi-classical Kroll-Watson model, which to date has been in agreement with (practically) all experiments. This implies that the atom's role is simply to balance momentum and is not "dressed" by the laser field. A simple model by Zon shows that any such dressing is dependent on the electric-dipole polarizability alpha of the target; for helium (alpha=1.4) dressing effects my be safely ignored. The first experiment to observe dressing effects was for xenon (alpha=28) by Morimoto et al. They observed effects that were qualitatively, but not quantitatively, in agreement with Zon's model. This disagreement may be due to experimental effects at the very small scattering angles used. The experiments to be carried out in the present project will be carried out in Potassium (alpha=290) for which Zon's model predicts large dressing effects at scattering angles readily accessible experimentally. The multipass laser system will use a Pockels cell to rotate the polarization of the laser pulse once it is injected into the system, thereby trapping it in a repetitive optical path that contains a polarizing beam-splitting cube.

该项目检查了暴露于激光光的电子的所谓免费过渡。在中性原子的气体中，结合的原子电子可能很容易发出或吸收光子（光的颗粒或电磁辐射）。然而，除了中性原子，离子和未结合（游离）电子外，在部分离子化的气体（血浆）中还存在。由于能量和动量的保护定律，这些自由电子只能在与原子发生碰撞时吸收（或发射）光子。该过程称为免费的过渡，即电子在碰撞之前是免费的，在碰撞之前是免费的，但是获得或损失了相当于存在的一个（或更多）光子的能量。已经表明，免费的免费工艺对天体物理等离子体的特性具有重大影响，例如恒星（和太阳能）大气层和星际空间中的物体，以及融合研究（融合反应堆）和照明工业（Florescent Tubes）中遇到的等离子体。因此，重要的是要对自由工艺有详细的了解，并且可以在实验室中通过在激光束的存在下从燃气射流中散射电子束来研究这些知识。迄今为止，除一个实验外，所有实验都发现免费过渡与使用该原子（或分子）无关。最近，在日本进行的一个实验发现了第一个实验证据表明原子类型会影响过渡。实验是用可观察到的效果的氙原原子进行的，但很小。该项目将在钾中进行类似的实验，其中一个简单的理论预测效果将大十倍。该项目还将开发所谓的多通系统，在该系统中，激光束（每秒每秒发射30次产生激光脉冲）将至少在镜子之间来回弹跳，因此将使实验能够在十分之一的时间内进行实验。该系统将使用特殊的“光学门”，该系统将允许激光脉冲被“注入”两个镜子之间的空间。自由过渡的目标独立性是简单的半古典kroll-watson模型的要求，迄今为止，该模型与所有实验都一致。这意味着原子的角色仅仅是为了平衡动力，而不是被激光领域“打扮”。 ZON的简单模型表明，任何此类敷料都取决于目标的电偶极极化性α。对于氦气（alpha = 1.4），敷料效果我被安全地忽略了。观察敷料效果的第一个实验是Morimoto等人的氙气（alpha = 28）。他们观察到与Zon的模型一致的质量上但不是定量的效果。这种分歧可能是由于在使用非常小的散射角度的实验效应所致。在本项目中要进行的实验将在钾（alpha = 290）中进行，ZON的模型可以在散射角度易于实验可访问较大的敷料效应。一旦将激光脉冲注入到系统中，多通激光系统将使用Pockels单元旋转激光脉冲的极化，从而将其捕获在重复的光学路径中，其中包含一个偏振光梁切开的立方体。