Electron Scattering from Fundamental Gaseous Targets

物质的电子散射导致了一系列现象，包括行星大气(包括地球大气)发出的光，汽油发动机的火花，以及暴露在辐射下的生物组织中DNA的碎片。这些过程可以用量子力学理论来理解。目前的实验项目涉及在一束定义明确的电子束与放置在其路径上的气体原子或分子之间建立受控碰撞。其目的是仔细研究这些电子是如何被目标气体原子或分子偏转的，以及它们如何在给定的电子能量下改变这些目标的物理和化学状态。这些实验针对大范围的电子能量进行，将观察电子和靶子之间的动态相互作用，并深入研究控制电子如何在这些受控能量下向不同方向散射的物理。目标包括简单的原子，如霓气、氩、氪和氙气，分子氢和氮，以及更复杂的分子，如水、醇和苯型芳香族分子。实验结果被用来测试详细的量子散射模型，以促进我们对这些电子与目标相互作用的理解。该项目在同一个实验室继续了先前的工作，促进了工业过程的建模。这个项目的一个好处是，由于本科生参与了这个项目，他们将有机会接触到世界范围的博士项目。一种新的电子飞行时间光谱仪将能够处理从散射事件中出现的非常慢的电子。电子束是脉冲的，散射的电子在速度上被它们到达探测器所需的时间分开。用电子能量损失谱研究了动能从0.5 eV到100 eV的低能电子的散射，其中1 mm高能量分辨率的准直光束的入射能量在真空室中穿过一束细小的纯原子或分子。电子(由钨丝源产生)的能量分离是在高分辨率(30-50 meV满-最大值为一半)下使用静电透镜和半球分析仪进行的。测量包括电子对稀有气体原子和简单双原子分子(H2和N2)的微分散射截面和极化关联。这些数据为电子散射模型提供了测试，并揭示了散射过程的量子动力学，其中涉及库仑相互作用、电子自旋过程(自旋交换、自旋轨道)和共振相互作用的细节。到目前为止，随着计算能力的迅速增加，模型一直在进化，以处理更复杂的目标。本项目将研究发射的真空紫外线辐射的极化，并垂直于散射面，与能量损失与辐射激发能量一致的差异散射电子相吻合。重要的是，它将测量真空紫外线中辐射的圆极化，这个参数与散射电子赋予目标的角动量有关，并为碰撞过程提供有价值的物理见解。此外，开发一种新的飞行时间光谱仪(使用快1纳秒脉冲电子束)将能够对散射截面进行绝对校准，并能够处理0.5 eV至20 eV能量范围内的缓慢涌现电子，并增加整个正在进行的测量的范围和精度。该项目旨在继续为实验室研究中的本科生提供准确的碰撞数据。

Electrons scattering from matter are responsible for a host of phenomena, including the light emitted from planetary atmospheres (including that of the Earth), the sparks in gasoline engines, and the fragmentation of DNA in biological tissues exposed to radiation. These processes can be understood by the theory of quantum mechanics. The present experimental project involves setting up controlled collisions between a well-defined beam of electrons and gas atoms or molecules placed in their path. The aim is to carefully investigate how these electrons are deflected by the target gas atoms or molecules and how they change the physical and chemical state of these targets for a given energy of the electrons. The experiments are conducted for a wide range of electron energies and will look at the dynamic interaction between the electrons and the targets and delve into the physics which controls how the electrons are scattered in various directions at these controlled energies. The targets include simple atoms such as neon, argon, krypton, and xenon, molecular hydrogen and nitrogen, and more complex molecules such as water, alcohols and benzene-type aromatic molecules. The experimental results are used to test detailed quantum scattering models to promote our understanding of the interaction of these electrons with targets. The project continues prior work in the same lab which has advanced the modeling of industrial processes. A benefit from this project will be the exposure of undergraduates to world-wide PhD programs as a result of their engagement in this project.A new electron time-of-flight spectrometer will be able to handle very slow electrons emerging from scattering events. The electron beam is pulsed and the scattered electrons are separated in velocity by the amount of time they take to reach the detector. The scattering of low energy electrons with kinetic energies ranging from 0.5 eV to 100 eV is studied using electron energy loss spectroscopy where the incident energy of a 1 mm collimated beam of high energy resolution crosses a tenuous beam of pure atoms or molecules in a vacuum chamber. The energy separation of electrons (produced from a tungsten filament source) is made at high resolution (30-50 meV full-with at half maximum) using electrostatic lenses combined with hemispherical analyzers. The measurements consist of differential scattering cross sections and polarization correlations for electron scattering from rare gas atoms and simple diatomic molecules (H2 and N2). The data provide tests for models of electron scattering and shed light on the quantum dynamics of the scattering process which involves details of Coulomb interactions, electron spin processes (spin-exchange, spin-orbit), and resonant interactions. Models to date have been evolving to handle more complex targets as computational power is rapidly increasing. The present project will look at the polarization of emitted vacuum ultraviolet radiation and, perpendicular to the scattering plane, in coincidence with differentially scattered electrons whose energy loss coincides with the excitation energy of the radiation. Importantly, it will measure the circular polarization of the radiation in the vacuum ultraviolet, a parameter which is related to the angular momentum imparted to the target by the scattered electron, and provides valuable physical insights to the collision process. In addition, the development of a new time-of-flight spectrometer (using a fast 1 nanosecond pulsed electron beam) will enable absolute calibration of scattering cross sections as well as be able to handle slow emergent electrons in the energy range of 0.5eV to 20eV, and add to the range and accuracy of the overall ongoing measurements. This project aims to continue its productive supply of accurate collision data involving undergraduates in laboratory research.