Photoemission electron microscopy (PEEM, also called photoelectron microscopy, PEM) 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 (LEED), and low-energy electron microscopy (LEEM). In biology, it is called photoelectron microscopy (PEM), which fits with photoelectron spectroscopy (PES), transmission electron microscopy (TEM), and scanning electron microscopy (SEM).
History
Initial development In 1933, Ernst Brüche reported images of cathodes illuminated by UV light. This work was extended by two of his colleagues, H. Mahl and J. Pohl. Brüche made a sketch of his photoelectron emission microscope in his 1933 paper (Figure 1). This is evidently the first photoelectron emission microscope (PEEM).
Improved techniques In 1963, Gertrude F. Rempfer designed the electron optics for an early ultrahigh-vacuum (UHV) PEEM. In 1965, G. Burroughs at the Night Vision Laboratory, Fort Belvoir, Virginia built the bakeable electrostatic lenses and metal-sealed valves for PEEM. During the 1960s, in the PEEM, as well as TEM, the specimens were grounded and could be transferred in the UHV environment to several positions for photocathode formation, processing and observation. These electron microscopes were used for only a brief period of time, but the components live on. The first commercially available PEEM was designed and tested by Engel during the 1960s for his thesis work under E. Ruska and developed it into a marketable product, called the "Metioskop KE3", by Balzers in 1971. The electron lenses and voltage divider of the PEEM were incorporated into one version of a PEEM for biological studies in Eugene, Oregon around 1970.
Further research During the 1970s and 1980s the second generation (PEEM-2) and third generation (PEEM-3) microscopes were constructed. PEEM-2 is a conventional not aberration-corrected instrument employing electrostatic lenses. It uses a cooled charge-coupled device (CCD) fiber-coupled to a phosphor to detect the electron-optical image. The aberration corrected microscope PEEM-3 employs a curved electron mirror to counter the lowest order aberrations of the electron lenses and the accelerating field.
Background
Photoelectric effect
The photoemission or photoelectric effect is a quantum electronic phenomenon in which electrons (photoelectrons) are emitted from matter after the absorption of energy from electromagnetic radiation such as UV light or X-ray. When UV light or X-ray is absorbed by matter, electrons are excited from core levels into unoccupied states, leaving empty core states. Secondary electrons are generated by the decay of the core hole. Auger processes and inelastic electron scattering create a cascade of low-energy electrons. Some electrons penetrate the sample surface and escape into vacuum. A wide spectrum of electrons is emitted with energies between the energy of the illumination and the work function of the sample. This wide electron distribution is the principal source of image aberration in the microscope.
Quantitative analysis
Using Einstein's method, the following equations are used: energy of photon = energy needed to remove an electron + kinetic energy of the emitted electron
h f = ϕ + E k max {\displaystyle hf=\phi +E_{k_{\text{max}}}\,}
where
h is the Planck constant; f is the frequency of the incident photon;
ϕ = h f 0 {\displaystyle \phi =hf_{0}\ } is the work function;
E k max = 1 2 m v m 2 {\displaystyle E_{k_{\text{max}}}={\tfrac {1}{2}}mv_{m}^{2}} is the maximum kinetic energy of ejected electrons; f0 is the threshold frequency for the photoelectric effect to occur; m is the rest mass of the ejected electron; vm is the speed of the ejected electron.
Electron emission microscopy Electron emission microscopy is a type of electron microscopy in which the information-carrying beam of electrons originates from the specimen itself. The energy source responsible for electron emission can be heat (thermionic emission), light (photoelectron emission), ions, or neutral particles, but typically excludes field emission and other methods involving point sources or tip-based microscopy.
Photoelectron imaging Photoelectron imaging includes any form of imaging in which the source of information is the distribution of points from which electrons are ejected from the specimen by the action of photons. The technique with the highest resolution photoelectron imaging is presently photoelectron emission microscopy using UV light.
Photoemission electron microscope A photoemission electron microscope is a parallel imaging instrument. It creates at any given moment a complete picture of the photoelectron distribution emitted from the imaged surface region.
Light sources The viewed area of the specimen must be illuminated homogeneously with appropriate radiation (ranging from UV to hard x-rays). UV light is the most common radiation used in PEEM because very bright sources are available, such as mercury lamps. However, other wavelengths (like soft x-rays) are preferred where analytical information is required.
Electron optical column and resolution
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