Theory-Driven Experimental Studies of Planar Photocathodes

点击翻译按钮获取中文摘要

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.