Polarized Electron Physics

电子具有“自旋”的基本性质，类似于旋转玩具的陀螺，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所起的作用。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。锌原子也被用作实验对象，以检验另一个研究小组最近做的一项实验的结果。在这项实验中，锌原子受到极化电子的激发，以一种所有已知的原子碰撞理论都禁止的方式发光。如果这一结果重现，许多关于自旋极化电子碰撞激发物理学的基本理论知识将需要修订。在这个项目中，改进的极化电子源也正在开发中，其目标是制造“交钥匙”组件，使其易于集成到其他实验系统中。这个项目正在研究制造极化电子的两种特殊方法。第一种是电子与铷原子的碰撞，其中自旋从铷转移到自由电子。第二种方法是利用砷化镓（GaAs）等半导体的多光子电离，使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为生物和材料研究以及工业提供新的分析工具。涉及极化电子和手性分子之间碰撞的实验将扩展先前的工作，这些工作显示了对卤脑靶的准弹性和反应性散射的手性敏感性。现在已经观察到这种敏感性，我们的目标是在具有生物学意义的分子中证明这种效应，如半胱氨酸，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。锌实验的目的是检查在激发锌原子的荧光中观察到的相当大的倾斜线性偏振值（偏振分数P2）是否可重复。没有现存的电子-原子散射理论，包括最先进的“伪态r矩阵”方法，能够证实这一结果，即使他们对该系统的其他碰撞参数做出了定量准确的预测。源开发工作将基于该小组成功演示的“Rb自旋滤波器”设计，其中光泵浦Rb与入射的非极化电子束进行自旋交换碰撞。本项目将重点改进真空系统、Rb靶的可靠性和光抽运协议。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GaAs实验中，来自飞秒Ti:Saph振荡器的未放大脉冲被用来发光电子。第一个实验将研究大块GaAs和尖状GaAs碎片发射的电子的强度和极化。然后考虑砷化镓尖端阵列的目标。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。

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 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 another 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 that result is reproduced, much of the basic theoretical knowledge about spin-polarized electron-impact-excitation physics would need to be revised. Improved sources of polarized electrons are also being developed in this project, with the goal of making "turnkey" components that will be easily integrated into other experimental systems. Two particular ways to make polarized electrons are being investigated in this project. 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 (GaAs) to give the electrons a preferential spin direction. This research 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. 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, the 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 Zn experiments is to check whether the rather large value of canted linear polarization (polarization fraction P2) observed in fluorescence from excited Zn 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 "Rb spin-filter" design by the group, in which optically-pumped Rb undergoes spin-exchange collisions with an incident unpolarized beam of electrons. This project will focus on improving the vacuum system, the Rb 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 Rb vapor. In the GaAs experiment, unamplified pulses from a femtosecond Ti:Saph oscillator are used to photo-emit electrons. First experiments will investigate the intensity and polarization of the electrons emitted from bulk GaAs and tip-like GaAs shards. Then targets of GaAs cusp arrays will be considered.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.