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

这个项目研究了暴露在激光下的电子的所谓自由-自由跃迁。在中性原子的气体中，束缚的原子电子可以很容易地发射或吸收光子（光的粒子或电磁辐射）。然而，在部分电离的气体（等离子体）中，除了中性原子外，还存在离子和未束缚的（自由）电子。由于能量和动量守恒定律，这些自由电子在与原子碰撞时只能吸收（或发射）一个光子。这个过程被称为自由-自由跃迁，也就是说，电子在碰撞之前是自由的，在碰撞之后也是自由的，但是获得或失去的能量等于一个（或多个）光子的能量。已经证明，自由-自由过程对天体物理等离子体的性质有重大影响，例如在恒星（和太阳）大气和星际空间中的等离子体，以及在聚变研究（聚变反应堆）和照明工业（荧光灯管）中遇到的等离子体。因此，掌握自由-自由过程的详细知识是很重要的，这些过程可以在实验室中通过在激光束存在的情况下从气体射流散射电子束来研究。迄今为止，除了一个实验外，所有的实验都发现自由-自由跃迁与使用哪个原子（或分子）无关。最近，在日本进行的一项实验首次发现了原子类型可以影响跃迁的实验证据。实验是用氙原子进行的，这种效应是可以观察到的，但很小。这个项目将在钾离子上进行类似的实验，在那里，一个简单的理论预测，效果将是钾离子的十倍。该项目还将开发所谓的多通道系统，其中激光束（每秒发射30次以产生激光脉冲）将在镜子之间来回反弹至少10次，因此将使实验在十分之一的时间内进行。该系统将使用一种特殊的“光学门”，允许激光脉冲在被捕获之前“注入”到两个镜子之间的空间。自由-自由跃迁的目标独立性是简单的半经典Kroll-Watson模型的要求，迄今为止（实际上）与所有实验一致。这意味着原子的作用仅仅是平衡动量，而不是被激光场“装扮”。Zon的一个简单模型表明，任何这种修饰都依赖于目标的电偶极极化率；对于氦（α =1.4）的修整效果可以安全地忽略。Morimoto等人首次观察了氙（α =28）的敷料效果。他们观察到的影响是定性的，而不是定量的，与Zon的模型一致。这种分歧可能是由于实验效应在非常小的散射角所使用的。在本项目中进行的实验将在钾（α =290）中进行，Zon的模型预测在易于实验获得的散射角上有很大的修饰效应。多通道激光系统将使用波克尔斯电池来旋转激光脉冲的偏振，一旦它被注入系统，从而将其困在一个包含偏振分束立方体的重复光路中。

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.