Polarized Electron Physics

电子具有“自旋”的基本性质，这类似于旋转的玩具陀螺，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-所有天然存在的DNA都以相同方向螺旋的事实。 锌原子也被用作目标，以检查最近由另一个小组完成的实验结果，其中锌原子被极化电子激发，以所有已知的原子碰撞理论所禁止的方式发射光。 如果这一结果得以重现，那么许多关于自旋极化电子碰撞激发物理学的基础理论知识都需要修正。该项目还在开发改进的极化电子源，目标是制造易于集成到其他实验系统中的“交钥匙”组件。 这个项目正在研究两种特殊的方法来制造极化电子。第一个是电子与铷原子的碰撞，在碰撞中，自旋从铷原子转移到自由电子。第二种方法是利用砷化镓（GaAs）等半导体的多光子电离来赋予电子优先的自旋方向。这项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。涉及极化电子和手性分子之间的碰撞实验将扩展以前的工作，表现出在准弹性和反应性散射与halocamphor目标的手性灵敏度。 既然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（如半胱氨酸）中证明这种效应，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。 Zn实验的目的是检查在来自激发的Zn原子的荧光中观察到的倾斜线性偏振（偏振分数P2）的相当大的值是否是可再现的。 没有现存的电子-原子散射理论，包括最先进的“R-矩阵与赝态”的方法，已经能够证实这一结果，即使他们作出定量准确的预测，该系统的其他碰撞参数。源开发工作将基于该小组成功展示的“Rb自旋过滤器”设计，其中光泵Rb与入射的非偏振电子束进行自旋交换碰撞。 该项目将集中在改进真空系统，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蒸汽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用于光发射电子。第一个实验将研究从体砷化镓和尖端状砷化镓碎片发射的电子的强度和极化。 该奖项反映了NSF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.