Polarized Electron Physics

电子具有“自旋”的基本性质，这类似于旋转的陀螺，并且与它们的角动量有关。这个项目研究极化电子和手性分子之间的碰撞，极化电子的自旋在一个方向上排列。 以DNA为例，这种分子的特征是螺旋形或螺旋几何形状。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-所有天然存在的DNA都以相同方向螺旋的事实。 最近开发的极化电子源称为铷自旋过滤器被用于这项工作中，它的优点是它是不敏感的手性目标分子的污染，困扰早期的实验使用极化电子源的基础上从砷化镓的光电发射。氢分子（非手性）也在研究中。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。该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相关效应。 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来赋予光发射电子优先的自旋方向。这项旨在开发极化电子技术的研究有望提供新的高分辨率分析工具，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。极化电子和手性分子之间的碰撞实验将扩展以前的工作，表明手性效应与卤樟脑目标。 我们的目标是现在证明这样的影响，具有生物学意义的分子，如半胱氨酸，胱氨酸，硒代半胱氨酸，并研究最大的目标核电荷和目标的手性中心的位置对我们观察到的手性不对称性的影响。 简单原子对电子的散射已经很好理解，但即使是最简单的分子（如H2）对电子散射的理论也处于起步阶段，特别是当人们考虑涉及电子激发的过程时。H2实验将集中在特定过程的激发阈值之上的能量区域，在那里理论特别困难，因为激发的分子和几乎没有能量的后退电子长时间强烈相互作用。这些研究将严格测试为这种碰撞而开发的新理论。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极化电子的新来源的想法。这项工作的目标是制造“快”的电子脉冲，即，其持续时间与产生它们的光脉冲的持续时间相当，因此它们可以对诸如化学反应和固体中的磁波运动等过程进行时间分辨。将进行实验，以了解载流的钨纳米结构在受到短光脉冲撞击时是否也能产生极化电子。这项工作，基于自旋霍尔效应，将提供自旋电子学领域和自由极化电子束的生产之间的桥梁。这个奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.