page_title: Photoemission electron microscopy
section_title: Electron optical column and resolution
hierarchical_section_title: Photoemission electron microscopy / Photoemission electron microscope / Electron optical column and resolution
context_page_description: Photoemission electron microscopy is a type of electron microscopy that utilizes local variations in electron emission to generate image contrast. The excitation is usually produced by ultraviolet light, synchrotron radiation or X-ray sources. PEEM measures the coefficient indirectly by collecting the emitted secondary electrons generated in the electron cascade that follows the creation of the primary core hole in the absorption process. PEEM is a surface sensitive technique because the emitted electrons originate from a shallow layer. In physics, this technique is referred to as PEEM, which goes together naturally with low-energy electron diffraction, and low-energy electron microscopy. In biology, it is called photoelectron microscopy, which fits with photoelectron spectroscopy, transmission electron microscopy, and scanning electron microscopy.
context_section_description: The electron optical column contains two or more electrostatic or magnetic electron lenses, corrector elements such as a stigmator and deflector, an angle-limiting aperture in the backfocal plane of one of the lenses.
  As in any emission electron microscope, the objective or cathode lens determines the resolution. The latter is dependent on the electron-optical qualities, such as spherical aberrations, and the energy spread of the photoemitted electrons. The electrons are emitted into the vacuum with an angular distribution close to a cosine square function. A significant velocity component parallel to the surface will decrease the lateral resolution. The faster electrons, leaving the surface exactly along the center line of the PEEM, will also negatively influence the resolution due to the chromatic aberration of the cathode lens. The resolution is inversely proportional to the accelerating field strength at the surface but proportional to the energy spread of the electrons. So resolution r is approximately:
  In the equation, d is the distance between the specimen and the objective, ΔE is the distribution width of the initial electron energies and U is the accelerating voltage.
  Besides the cathode or objective lens, situated on the left hand side of Figure 4, two more lenses are utilized to create an image of the specimen: an intermediate three-electrode lens is used to vary the total magnification between 100× if the lens is deactivated, and up to 1000× when needed. On the right-hand side of Figure 4 is the projector, a three electrode lens combined with a two-element deceleration lens. The main task of this lens combination is the deceleration of the fast 20 keV electrons to energies for which the channelplate has its highest sensitivity. Such an image intensifier has its best performance for impinging electrons with kinetic energies roughly about 1 keV.
caption: Typical photoemission electron microscope
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