Theory-Driven Experimental Studies of Planar Photocathodes

产生良好准直的（即，明亮的）脉冲电子束是当今（和未来提出的）许多尖端研究仪器中的关键部件，这些尖端研究仪器旨在提高我们对材料、纳米级系统和分子动力学在快速时间尺度上和具有高空间分辨率的理解。 该研究项目将建立在我们最近开发的理论形式主义的基础上，并验证我们最近开发的理论形式主义，该理论形式主义描述了这些电子脉冲的特性如何从根本上取决于电子发射（光阴极）材料的电子特性，其目标是显着提高它们的亮度。 预期相关研究仪器（如直线加速器相干光源和具有快速时间分辨率的电子显微镜）的性能将随之增强，这将导致切实的科学进步，特别是在可持续能源研究、分子生物化学和材料工程等具有战略重要性的领域。 除了教育加速器科学，凝聚态物理学和光子学的下一代领导者外，该项目还将使STEM本科生接触高度跨学科的现代研究工作。 这个为期三年的研究项目的目标是使用基于能带结构的光电发射理论来识别产生超低发散电子束的稳健平面光阴极材料，从而大幅提高激光驱动电子源的束流质量。 先前的实验和理论工作相结合，提供了一个框架，更深入地了解平面光电阴极的物理，现在将允许一个预测的方法来选择光电阴极。 实验验证后的密度泛函理论（DFT）的方法来模拟光电发射使用单晶金属，进一步的研究将进行光阴极材料选择其大幅减少电子束发散与预期的低值的均方根横向动量，PT，发射的电子。 对理论预测的评估将采用螺线管扫描技术来测量使用激光驱动直流电子枪产生的电子束的pT。 所有实验都将使用定制的230- 300 nm可调谐紫外激光辐射源进行，以允许原位验证功函数和定量光阴极特性的预测光谱依赖性，包括量子发射效率。 这也将需要一个扩展的DFT为基础的光电发射理论，包括有限的温度效应和矩阵元素描述的光阴极材料中的光激发。 此外，调查的重要影响的表面粗糙度的平面金属和半导体光电阴极的发射性能，并没有直接的一步光激发体态发射将启动。

The generation of well-collimated (i.e., bright) pulsed electron beams is a key component in many of todays (and proposed future) cutting edge research instruments aimed at improving our understanding of materials, nanoscale systems, and molecular dynamics on fast timescales and with high spatial resolution. This research project will build on and verify our recently developed theoretical formalism describing how the characteristics of these electron pulses are fundamentally dependent upon the electronic properties of the electron-emitting (photocathode) material, with the goal of significantly increasing their brightness. The consequent expected enhancement in the performance of the related research instrumentation (such as the Linac Coherent Light Source and electron microscopes incorporating fast time resolution) should result in tangible scientific advances, particularly in the strategically important fields of sustainable energy research, molecular biochemistry, and materials engineering. In addition to educating the next generation of leaders in accelerator science, condensed matter physics, and photonics, this project will also expose STEM undergraduate students to highly interdisciplinary modern research work. The goal of this three-year research project is to improve substantially the beam quality of laser-driven electron sources using a band structure based photoemission theory to identify robust planar photocathode materials that generate electron beams with ultra-low divergence. Prior combined experimental and theoretical work has provided the framework for a deeper understanding of the physics of planar photocathodes that will now allow for a predictive approach to photocathode selection. After experimental verification of the density functional theory (DFT) approach to the simulation of photoemission using single-crystal metals, further studies will be conducted on photocathode materials selected for their substantially reduced electron beam divergence associated with expected low values of the rms transverse momentum, pT, of the emitted electrons. Assessment of the theoretical predictions will employ the solenoid scan technique to measure pT of electron beams generated using a laser-driven DC electron gun. All experiments will be performed using a custom-built, 230-300nm tunable ultraviolet laser radiation source to allow in situ verification of the work function and the predicted spectral dependence of quantitative photocathode properties, including quantum emission efficiency. This will also require an extension to the DFT-based photoemission theory to include finite temperature effects and matrix elements describing the photoexcitation in the photocathode material. In addition, an investigation into the important effect of surface roughness on the emission properties of planar metal and semiconductor photocathodes with and without direct one-step photoexcited bulk state emission will be initiated.