Electron spectrometer

In an electron spectrometer, an incoming beam of electrons is bent with electric or magnetic fields. As higher energy electrons will be bent less by the beam, this produces a spatially distributed range of energies.

Electron spectrometers are used on a range of scientific equipment, including particle accelerators, transmission electron microscopes, and astronomical satellites.

Types

An electrostatic electron spectrometer uses the electric field, which cause electrons to move along field gradients, whereas magnetic devices cause electrons to move at right angles to the field. Magnetic fields will act in a direction perpendicular to the electron propagation, thereby conserving velocity, whereas electrostatic fields will cause electrons to move along the field gradient,[1] which may change electron energies if the component of the direction of propagation and field gradients are not perpendicular. Owing to these effects, sector based designs are commonly used in electron spectrometers.

Construction

The effective potential in the solution of motion in a magnetic or electric system with rotational symmetry leads to radial focusing onto a mean radius.[1] By superposition of a quadrupole field axial focusing is possible while weakening the radial focusing, until the astigmatism vanishes. By breaking the rotational symmetry a bit and varying the electrostatic potential along the mean path of the spherical aberration is minimized.

The spectrometer can use entrance and exit slits or use a small source, which only emits into specific angle and a small detector. Photoelectron spectra from single crystals exhibit a dependency on the emission angle, so that the entrance slit is needed. All the electrons from an isotopic source may be sucked off and focused into a directed beam (much like in an electron gun), which can then be analyzed. A position sensitive detector can detect the energy along one direction and depending on the additional optics lateral resolution or one angle along the other direction.

Electrostatic spectrometers preserve the spin, which can be resolved afterwards.

References

  1. 1 2 Zworykin,V ; Morton, G ; Ramberg, E ; Hillier J; Vance A (1945). Electron optics and the electron microscope. John Wiley and Sons, New York.

See also

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