Polarized Electron Physics

电子具有“自旋”的基本性质，这类似于旋转的玩具顶部，并与它们的角动量有关。这个项目研究极化电子之间的碰撞，极化电子的自转方向是一个方向，而所谓的“手性”或“手性”分子(这类分子，以DNA为例，以螺旋或螺旋几何为特征)之间的碰撞。这些实验解决了有关电子-手性分子散射动力学的物理问题，特别是关于电子自旋所起的作用。它们还将提供关于生物同性起源的重要线索--所有自然发生的DNA都朝着同一方向螺旋的事实。锌原子也被用作靶子，以检查一个澳大利亚小组最近进行的一项实验的结果。在该实验中，被极化电子激发的锌原子以一种所有已知的原子碰撞理论都禁止的方式发光。如果复制澳大利亚的结果，我们在这个问题上的许多基本理论知识将被证明是错误的。改进的极化电子源也在开发中，目标是“交钥匙”，方便使用。这个项目关注两种制造极化电子的特殊方法：第一种是电子与Rb原子的碰撞，在这种碰撞中，自旋从Rb转移到自由电子。第二种方法是利用砷化镓等半导体的多光子电离，为电子提供一个优先的自旋方向。这项旨在开发极化电子技术的研究有望为生物和材料研究以及工业提供新的分析工具。这些涉及极化电子与手性分子碰撞的实验将扩展先前的工作，即在准弹性散射和反应散射中都显示出对卤水目标的手性敏感性。既然已经观察到了这种敏感性，我们的目标是在具有生物学意义的分子中展示这种效应，例如半胱氨酸，并研究最大靶核电荷和靶手性中心位置对我们观察到的手性不对称性的影响。锌实验的目的是检查在被激发的锌原子的荧光中观察到的相当大的斜线性极化值(极化分数P2)是否可重现。现有的电子-原子散射理论，包括最先进的“具有赝态的R矩阵”方法，都不能证实这一结果，尽管它们对该系统的其他碰撞参数进行了准确的定量预测。源开发工作将基于我们小组成功演示的“Rb自旋滤光器”设计，在该设计中，光泵浦的Rb与入射的未极化电子束发生自旋交换碰撞。该项目将致力于改进真空系统，提高Rb靶的可靠性，并改进光泵浦方案。它还将研究各种缓冲气体，以了解电子束和Rb蒸气之间相互作用的复杂物理。在砷化镓实验中，来自飞秒钛蓝宝石振荡器的未放大脉冲被用来光电发射电子。首先，实验将研究从块状砷化镓和尖状砷化镓碎片中发射的电子的强度和极化。然后将考虑砷化镓尖点阵列的靶材。

Electrons have the fundamental property of "spin," which is analogous to that of a spinning toy top, and is associated with their angular momentum. This project studies collisions between polarized electrons, which have their spins aligned in one direction, and so-called "chiral", or "handed" molecules (such molecules, of which DNA is an example, are characterized by a spiral, or helical geometry). These experiments address physics questions about the dynamics of electron-chiral molecule scattering, particularly with regard to the role played by the electron spins. They will also provide important clues about the origins of biological homochirality--the fact that all naturally-occurring DNA spirals in the same direction. Atoms of zinc are also being used as targets, to check the results of an experiment done recently by an Australian group in which zinc atoms excited by polarized electrons emitted light in a way that is forbidden by all known theories of atomic collisions. If the Australian result is reproduced, much of our basic theoretical knowledge on this topic will be shown to be in error. Improved sources of polarized electrons are also being developed, with the goal of "turnkey" ease of use. This project focuses on two particular ways to make polarized electrons: the first involves electron collisions with rubidium atoms in which spin is transferred from the rubidium to the free electrons. The second uses multiphoton ionization of semiconductors such as gallium arsenide to give the electrons a preferential spin direction. This research done to develop polarized electron technology holds the promise of providing new analytical tools that can be used for biological and materials research, and for industry. The experiments involving collisions between polarized electrons and chiral molecules will extend previous work that showed chiral sensitivity in both quasi-elastic and reactive scattering with halocamphor targets. Now that such sensitivity has been observed, our goal is to demonstrate such effects in molecules that have biological significance, such as cysteine, and to study the effect of the maximum target nuclear charge and location of the target's chiral center on the chiral asymmetries we observe. The goal in the zinc experiments is to check whether the rather large value of canted linear polarization (polarization fraction P2) observed in fluorescence from excited zinc atoms is reproducible. No extant theory of electron-atom scattering, including the state-of-the-art "R-matrix with pseudostates" approach, has been able to confirm this result, even though they make quantitatively accurate predictions of other collision parameters for this system. Source development work will be based on the successfully demonstrated "rubidium spin-filter" design by our group, in which optically-pumped rubidium undergoes spin-exchange collisions with an incident unpolarized beam of electrons. This project will focus on improving the vacuum system, the rubidium target reliability, and the optical pumping protocol. It will also study a variety of buffer gases to understand the complex physics of the interaction between the electron beam and the rubidium vapor. In the gallium arsenide experiment, unamplified pulses from a femtosecond titanium-sapphire oscillator are used to photo-emit electrons. First experiments will investigate the intensity and polarization of the electrons emitted from bulk gallium arsenide and tip-like gallium arsenide shards. Then targets of gallium arsenide cusp arrays will be considered.