High-Resolution EELS in an aberration-corrected LEEM: Design of electrostatic transfer lenses for hemispherical filters

Recent advances in Low-Energy Electron Microscopy (LEEM), including the development of aberration-corrected (AC) systems, have significantly enhanced spatial resolution. However, further progress is limited by the energy resolution of current instruments. In this work, we propose a novel approach to address this limitation by integrating two Hemispherical Deflector Analyzers (HDAs) in tandem: the first serving to monochromatize the electron source, and the second to enhance the spectroscopic performance of AC-LEEM for Electron Energy Loss Spectroscopy (EELS). This dual-HDA configuration provides a clear pathway toward combining high spatial and energy resolution, expanding the capabilities of LEEM for advanced surface and materials characterization. This paper discusses various criteria for implementing these HDAs on a commercial AC-LEEM and presents more specifically the design of four transfer lenses (TLs) for electron transfer between the HDAs and the other optical components of the instrument. The use of a natural aberration correction scheme based on the dispersion-compensation (DC) principle is also discussed for maximum current throughput. Using ray-tracing simulations, we first show that the TL design can effectively decelerate/accelerate the electrons between 0.1 and 15 keV, thus respecting the high-voltage operation of the AC-LEEM. A double focus of the electron beam is also simulated so that the electron positions are conserved after transfer at the exit/entrance of the HDAs, an important condition for DC operation. Finally, ray-tracing simulations of the TLs show that the focal plane can be switched from the image plane to the back focal plane, allowing fast switching between diffraction and imaging modes.