An RF cavity-based ultrafast electron energy loss spectrometer: A new tool for unraveling dynamic processes in materials

电子能量损失谱（EELS）是一种非常强大和发展良好的技术，对材料物理研究非常有价值，它提供了许多低能（0 - 10 eV）基元激发的光谱信息，如价层电子激发，声子，等离子体激元和所有波矢的电荷密度振荡。也可以测量高能量损失激发（10 - 1000 eV），如核心电子激发，提供类似于X射线吸收光谱的信息，对基础研究非常重要的吸收物质的价态，局部几何形状和键合。然而，传统的EELS只能研究处于平衡状态的材料。在这里，我们建议构建一种新的动量分辨超快电子能量损失谱（q-uEELS）仪器，无缝集成与现有的超快电子衍射仪在Siwick实验室。这款q-uEELS光谱仪利用飞行时间几何结构中的RF腔，实现能量和时间的高分辨率，提供具有飞秒至皮秒时间分辨率的能量损失光谱。q-uEELS提供的新测量能力将有助于揭示光致结构变化对材料特性的影响，以及材料中光激发的动力学对长度（subA-nm），时间（fs-ps）和能量（meV-eV）的自然尺度的影响，同时揭示先前隐藏的非弹性电子散射中包含的特征激发信息。 该提案利用了Siwick小组的两项最新进展，代表了一个令人兴奋的新科学机会，可以开发EELS来研究远离平衡的材料特性。

Electron energy loss spectroscopy (EELS) is an extremely powerful and well developed technique that has become extremely valuable for materials physics studies, providing spectroscopic information on a number of low energy (0 – 10eV) elementary excitations like valence shell electron excitations, phonons, plasmons and charge density oscillations of all kinds at all wavevectors. High-energy loss excitations (10 – 1000 eV) like core-electron excitations can also be measured, providing information similar to xray absorption spectroscopy on valency, local geometry and bonding of the absorbing species that has been so important for fundamental studies. Conventional EELS, however, is only capable of studying materials at equilibrium. Here we propose to construct a novel momentum-resolved ultrafast electron energy loss spectroscopy (q-uEELS) instrument that seamlessly integrates with the existing ultrafast electron diffractometer in the Siwick lab. This q-uEELS spectrometer makes use of RF cavities in a time-of-flight geometry to enable high resolution in both energy and time, providing energy loss spectra with a time resolution of femtoseconds to picoseconds. The new measurement capabilities provided by q-uEELS will help uncover the effects of photoinduced structural changes on material properties and the dynamics of optical excitation in materials on their natural scales of length (subA - nm), time (fs–ps) and energy (meV - eV) simultaneously by revealing the previously hidden information about characteristic excitations contained in the inelastic electron scattering. This proposal leverages two very recent advances in the Siwick group and represents an exciting new scientific opportunity to develop EELS for studies of material properties far from equilibrium.