Laser-Assisted Inelastic Electron-Atom Scattering

自由-自由散射是理解各种天体物理现象的重要基础过程。这个过程的命名是因为当一个电子在光子场的存在下从原子散射时，电子可以吸收或发射一个或多个光子（光粒子），但电子在散射碰撞之前和之后都是自由的。迄今为止，几乎所有的实验都表明，光子只与自由电子相互作用，对靶原子没有影响。然而，已经表明，所谓的“缀饰原子”效应（光子场与目标原子的相互作用）可以大大提高自由电子在非常小的电子散射角下吸收或发射光子的概率。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（类似于旧电视机中使用的电子枪）产生定义明确的电子束，两束电子束相互交叉，形成原子束。 该项目的一个重要部分是寻找氦中的“缀饰原子”效应，因为散射电子也改变了氦原子的能量状态。这项工作将对社区产生广泛的影响，为学生提供教育机会，在高中，本科生，研究生水平的肯塔基州的EPSCoR和合作，与唯一的本科机构伊利诺伊州卫斯理大学。这个项目涉及激光辅助电子散射的基础研究。这项工作将在两台设备上进行。第一个，这是已经在使用这样的实验，将被用来测量的“敷料”的最低激发态的He原子的1.17 eV的激光辐射在非弹性电子散射的影响。该装置的电子枪将装有单色仪，以解决紧密间隔的He 1 s2 s和1 s2 p单重态水平;只有前者预计将显示修整效应，重要的是要消除后者的大散射截面的影响。该设备还将与内部开发的多程激光系统接口，以提高自由-自由计数率;迄今为止，这种系统仅在国家实验室中使用，从未在“桌面”实验中使用。多程系统使用普克尔斯盒在注入后将线性偏振激光束旋转90度，结合分束立方体，然后将激光束捕获到重复路径中（包括现在停用的普克尔斯盒）。第二个装置最近已经装配了这样的多程激光系统。该装置将用于研究氦原子的激光辅助电子碰撞自电离。激光辅助自电离的预期特征是在喷射电子光谱中的边带，一个光子能量远离自电离共振位置。这种边带已经被其他研究人员在俄歇衰变中观察到，但从未在自电离过程中观察到。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。

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.