Laser-Assisted Inelastic Electron-Atom Scattering

自由-自由散射是理解各种天体物理现象的一个重要的基本过程。这一过程之所以被命名，是因为当一个电子在光子场存在的情况下从一个原子散射时，该电子可以吸收或发射一个或多个光子(光粒子)，但该电子在散射碰撞前后都是自由的。到目前为止，几乎所有的实验都表明，光子只与自由电子相互作用，对目标原子没有影响。然而，已有研究表明，在极小的电子散射角下，所谓的“修饰原子”效应(光子场与靶原子的相互作用)可以极大地提高自由电子吸收或发射光子的几率。该项目在实验室中研究自由自由散射，方法是使用激光产生定义明确的光子束，使用电子枪(类似于过去在旧电视机中找到的电子枪)产生定义明确的电子束，将两束光束安排为彼此相交和一束原子束。该项目的一个重要部分是寻找氦中的“修饰原子”效应，以寻找散射电子也会改变氦原子的能态的特殊情况。这项工作将对社区产生广泛的影响，为肯塔基州EPSCoR州的高中生、本科生和研究生提供教育机会，并与专门的本科生机构伊利诺伊州卫斯理大学合作。该项目涉及激光辅助电子散射的基础研究。这项工作将在两台仪器上进行。第一个已经用于此类实验，将用于测量1.17 eV激光辐射在非弹性电子散射过程中对氦原子最低激发态的“修饰”效应。本装置的电子枪将配备单色计，以分辨密集的He_1S_2S和1S_2P单重态能级，只有前者才能显示修饰效应，而消除后者的大散射截面的影响是重要的。该仪器还将与内部开发的多通道激光系统相连接，以提高自由-自由计数率；到目前为止，此类系统仅在国家实验室使用，从未用于“桌面”实验。多程系统在注入后使用普克尔斯盒将线偏振激光光束旋转90度，并结合分束立方体，然后将激光捕获到重复路径(包括现已停用的普克尔斯盒)。第二个装置最近安装了这样的多程激光系统。该装置将被用来研究激光辅助电子碰撞氦的自电离。激光辅助自电离的预期特征是发射电子光谱中的边带，距离自电离共振位置只有一个光子能量。这种边带在俄歇衰变中曾被其他工作人员看到，但在自动电离过程中从未被观察到。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Free-free scattering is a fundamental process that is important for the understanding of various astrophysical phenomena. The process is named for the fact that when an electron scatters from an atom in the presence of a photon field, the electron can absorb or emit one or more photons (particles of light), but the electron is free both before and after the scattering collision. Almost all experiments to date indicate that the photon interacts only with the free electron and has no effect on the target atom. However, it has been shown that a so-called "dressed atom" effect (the interaction of the photon field with the target atom) can greatly enhance the probability of the free electron absorbing or emitting a photon for very small electron scattering angles. This project investigates free-free scattering in the laboratory by using a laser to produce a well-defined photon beam, an electron gun (similar to those that used to be found in old TV sets) to produce a well-defined electron beam, with the two beams arranged to intersect each other and a beam of atoms. An important part of the project is to look for "dressed atom" effects in helium, for the special case where the scattered electron also changes the energy state of the helium atom. This work will have a broad impact on the community by providing educational opportunities for students at the high school, undergraduate, and graduate levels in the EPSCoR state of Kentucky and in collaboration with the exclusively undergraduate institution Illinois Wesleyan University.This project concerns fundamental research on laser-assisted electron scattering. The work will be carried out on two apparatuses. The first, which is already in use for such experiments, will be used to measure the effects of the "dressing" of the lowest excited states of He atoms by 1.17 eV laser radiation during inelastic electron scattering. The electron gun of this apparatus will be fitted with a monochrometer in order to resolve the closely spaced He 1s2s and 1s2p singlet levels; only the former is expected to show dressing effects and it is important to eliminate the effect of the large scattering cross section of the latter. The apparatus will also be interfaced with an in-house developed multipass laser system to increase the free-free count rate; to date such systems have only been used in national labs, never in "tabletop" experiments. The multipass system uses a Pockels cell to rotate the linearly polarized laser beam 90 degrees after injection, combined with a beam splitting cube that then traps the laser beam into a repetitive path (that includes the now-deactivated Pockels cell). The second apparatus has recently been fitted with such a multipass laser system. This apparatus will be used to investigate laser-assisted electron-impact autoionization of helium. The expected signatures of laser assisted autoionization are sidebands in the ejected-electron spectrum, one photon energy away from the autoionizing resonance positions. Such sidebands have been seen by other workers in Auger decay, but have never been observed for the autoionization process.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.