Polarized Electron Physics

极化电子物理

• 批准号: 2110358
• 负责人: Timothy Gay
• 金额: $ 68.99万
• 依托单位: University of Nebraska-Lincoln
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-09-01 至 2024-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2110358&HistoricalAwards=false
• 关键词: Polarized; Electron; Physics

Electrons have the fundamental property of “spin,” which is analogous to that of a spinning top, and is associated with their angular momentum. This project studies collisions between polarized electrons, which have their spins aligned in one direction, and 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 magnetic effects caused 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. A recently developed source of polarized electrons called a rubidium spin filter is being used in this work; it has the advantage that it is insensitive to the contamination by chiral target molecules that has plagued earlier experiments using polarized electron sources based on photoemission from gallium arsenide. Molecules of hydrogen (that are not chiral) are also being studied. Hydrogen is the simplest molecule, but its interaction with very slow electrons is poorly understood. This project will use very slow electrons that have very well defined energy and that are spin polarized to provide stringent tests of theory calculations that consider spin-dependent effects in such fundamental collisions. Improved sources of polarized electrons are also being developed that use multiphoton ionization of very sharp semiconductor tips to give the photoemitted electrons a preferential spin direction. This research done to develop polarized electron technology holds the promise of providing new, high resolution analytical tools that can be used for biological and materials research, and for industry. Graduate and undergradute students will develop their scientific abilities by their involvement in the research as will the general public though education outreach by the scientific team. The experiments involving collisions between polarized electrons and chiral molecules will extend previous work that showed chiral effects with halocamphor targets. The goal is to now demonstrate such effects in molecules that have biological significance, such as cysteine, cystine, and selenocysteine, 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 scattering of electrons by simple atoms is well understood, but the theory for electron scattering by even the simplest molecules such as H2 is in its infancy, especially when one considers processes involving electronic excitation. The H2 experiments will focus on the energy region just above the excitation threshold for specific processes, where the theory is particularly difficult, because the excited molecule and the receding electron, which has almost no energy, interact strongly for a long time. These studies will rigorously test new theories being developed for such collisions. Ideas for novel sources of polarized electrons based on multiphoton ionization of semiconducting nanotips and metallic nanostructures will be studied. The goal of this work is to make pulses of electrons that are “fast,” i.e., that have a duration comparable to that of the light pulses producing them, so they can time-resolve processes such as chemical reactions and magnetic wave motion in solids. Experiments will be done to learn if current-carrying nanostructures of tungsten can also produce polarized electrons when struck by short light pulses. This work, based on the spin Hall effect, will provide a bridge between the field of spintronics and the production of free polarized electron beams.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

电子具有“自旋”的基本性质，类似于旋转的陀螺，并与它们的角动量有关。这个项目研究了自旋在一个方向上的极化电子与手性或“手性”分子之间的碰撞。这样的分子，DNA就是一个例子，具有螺旋或螺旋几何的特征。这些实验解决了关于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引起的磁效应。它们还将提供有关生物同手性起源的重要线索，即所有自然产生的DNA都沿同一方向螺旋。在这项工作中使用了最近开发的一种称为铷自旋滤波器的极化电子源；它的优点是对手性靶分子的污染不敏感，这种污染一直困扰着早期使用基于砷化镓光电的极化电子源的实验。氢分子（非手性）也在研究之中。氢是最简单的分子，但人们对它与非常慢的电子的相互作用知之甚少。这个项目将使用非常慢的电子，这些电子具有非常明确的能量，并且是自旋极化的，以提供严格的理论计算测试，这些理论计算考虑了这种基本碰撞中的自旋依赖效应。改进的极化电子源也正在开发中，它使用非常尖锐的半导体尖端的多光子电离，使光电子具有优先的自旋方向。这项发展极化电子技术的研究有望提供新的、高分辨率的分析工具，可用于生物和材料研究以及工业。研究生和本科生将通过参与研究来发展他们的科学能力，公众也将通过科学团队的教育推广来发展他们的科学能力。涉及极化电子和手性分子之间碰撞的实验将扩展先前显示卤脑靶手性效应的工作。现在的目标是在具有生物学意义的分子中证明这种作用，如半胱氨酸、胱氨酸和硒型半胱氨酸，并研究最大靶核荷数和靶手性中心位置对我们观察到的手性不对称的影响。简单原子对电子的散射是很容易理解的，但是即使是像H2这样最简单的分子对电子散射的理论也处于起步阶段，特别是当人们考虑到涉及电子激发的过程时。H2实验将集中在特定过程的刚好高于激发阈值的能量区域，这是理论特别困难的地方，因为激发态分子和后退的电子（几乎没有能量）会长期强烈地相互作用。这些研究将严格检验为此类碰撞而发展起来的新理论。将研究基于半导体纳米尖端和金属纳米结构的多光子电离的新型极化电子源。这项工作的目标是制造“快速”的电子脉冲，即具有与产生它们的光脉冲相当的持续时间，因此它们可以时间分辨诸如化学反应和固体中的磁波动等过程。实验将研究载流钨的纳米结构在受到短光脉冲撞击时是否也能产生极化电子。这项基于自旋霍尔效应的工作，将为自旋电子学领域和自由极化电子束的产生提供一座桥梁。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

项目成果

A high-resolution, variable-energy electron beam from a Penning–Malmberg (Surko) buffer-gas trap

• DOI: 10.1140/epjd/s10053-022-00349-y
• 发表时间: 2022-02
• 期刊: The European Physical Journal D
• 作者: J. Machacek;T. Gay;S. Buckman;S. Hodgman

An improved source of spin-polarized electrons based on spin exchange in optically pumped rubidium vapor

基于光泵铷蒸气中自旋交换的改进自旋极化电子源

• DOI: 10.1063/5.0149691
• 发表时间: 2023
• 期刊: Review of Scientific Instruments
• 影响因子: 1.6
• 作者: Ahrendsen, K. J.;Trantham, K. W.;Tupa, D.;Gay, T. J.
• 通讯作者: Gay, T. J.

A Method to Measure Positron Beam Polarization Using Optically Polarized Atoms

一种利用光学偏振原子测量正电子束偏振的方法

• DOI: 10.3390/atoms11040065
• 发表时间: 2023
• 期刊: Atoms
• 影响因子: 1.8
• 作者: Machacek, Joshua R.;Hodgman, Sean;Buckman, Stephen;Gay, T. J.
• 通讯作者: Gay, T. J.