Low-energy Electron Microscopy - Introduction

Introduction

LEEM differs from conventional electron microscopies in four main ways:

  1. The sample must be illuminated on the same side of the imaging optics, i.e. through the objective lens, because samples are not transparent to low-energy electrons.
  2. In order to separate the incident and elastically scattered low energy electrons, scientists use magnetic “electron prism” beam separators which focus electrons both in and out of the plane of the beampath (to avoid distortions in the image and diffraction patterns).
  3. Rather than have the electron gun at a high potential and the rest of the instrument (including electron lenses and specimen) at ground, an electrostatic immersion objective lens brings the sample close to that of the gun, slowing down the high energy electrons to a desired energy only just before interacting with the sample surface.
  4. The instrument must be able to work under ultra-high vacuum (UHV), or 10−10 torr (760 torr = 1 atm, atmospheric pressure).

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