Fotokatoden

Fotokatoden (photocathode) is a specialized electron-emissive surface used in vacuum electronic devices. It consists of a material whose surface emits electrons when illuminated by photons, via the photoelectric effect. When photons with energy hν exceed the work function φ of the surface, electrons are liberated into vacuum and can be collected by an accelerating field. The emitted current increases with light intensity and, for many fotokatoden, with quantum efficiency that depends on wavelength and material.

Common fotokatoden materials include alkali-based compounds such as cesium telluride (Cs2Te) and multi-alkali Cs3Sb/K2CsSb, as well as GaAs-based cathodes activated with cesium and oxygen. These materials are often prepared as thin films on glass or metal substrates and operated under ultra-high vacuum. Some fotokatoden exhibit negative electron affinity, which lowers the surface barrier and improves emission.

Fotokatoden have spectral sensitivity from the ultraviolet to visible and, for specialized GaAs-type cathodes, into the near-infrared. Quantum efficiency ranges from well below 1% up to several tens of percent depending on wavelength, material, and activation. They require clean, stable surfaces and are susceptible to degradation from contamination, rough handling, and residual gases, limiting lifetime.

Applications include photomultiplier tubes and image intensifier tubes, scientific instruments such as electron microscopes, and certain X-ray detectors. Advantages include high gain, fast response, and low noise; limitations include vacuum dependence, finite lifetime, and sensitivity to environmental conditions.

See also: photoelectric effect, photocathode materials, photomultiplier tube.