An Aberration Corrected Photoemission Electron Microscope at the Advanced Light Source

Design of a new aberration corrected Photoemission electron microscope PEEM3 at the Advanced Light Source is outlined. PEEM3 will be installed on an elliptically polarized undulator beamline and will be used for the study of complex materials at high spatial and spectral resolution. The critical components of PEEM3 are the electron mirror aberration corrector and aberration-free magnetic beam separator. The models to calculate the optical properties of the electron mirror are discussed. The goal of the PEEM3 project is to achieve the highest possible transmission of the system at resolutions comparable to our present PEEM2 system (50 nm) and to enable significantly higher resolution, albeit at the sacrifice of intensity. We have left open the possibility to add an energy filter at a later date, if it becomes necessary driven by scientific need to improve the resolution further. INTRODUCTION X-ray excited photoemission electron microscope (PEEM) combines the power of modern synchrotron radiation absorption spectroscopy with direct full imaging capability of PEEM. A conventional PEEM system consists of round lenses whose resolution is limited by spherical and chromatic aberration, and the spherical aberration coefficient Cs and the chromatic aberration coefficient Cc always being positive [1]. As a result aberrations can only be minimized by adjusting the geometry of the electrodes but not eliminated. These aberrations limit the ultimate resolution, defined by a 50% value of the Modulation Transfer Function to be typically 50 – 100 nm, for x-ray illumination and a 20 KV extraction field [2]. Aberrations must be compensated in order to remove their deleterious effects on the imaging properties of the microscope. An electron mirror can have aberration coefficients of opposite sign but equal magnitude with respect to those of electron round lens so that in principle, the aberrations can be canceled out and the resolution can be improved ultimately to the diffraction limit. A new X-ray PEEM with an electron mirror aberration corrector at the ALS has been designed (PEEM3) and the overall layout and correction scheme are described.