fotemissio

Fotemissio, or photoemission, is the ejection of electrons from a material, typically a solid surface, as a result of absorbing electromagnetic radiation. It is most readily observed when the photon energy exceeds the material’s work function, the minimum energy needed to release an electron into vacuum. In metals this process is described by Einstein’s photoelectric equation: hν = φ + KEmax, where hν is the photon energy, φ the work function, and KEmax the maximum kinetic energy of the emitted electrons.

The emitted electrons form a photoelectron spectrum that carries information about the electronic structure of the material. The angular and energy distribution of the emitted electrons depends on factors such as the photon energy, polarization, and the surface orientation. Angle-resolved photoemission spectroscopy (ARPES) is a key technique that uses this process to map electronic band structures of solids.

Types and applications. External photoemission refers to electrons emitted into vacuum, a principle behind photocathodes used in electron sources for accelerators and detectors. Internal photoemission describes electron excitation and emission across a potential barrier within a device, such as at a metal–semiconductor interface, and underpins certain photodetectors. Materials chosen for photocathodes include alkali metals and compounds like GaAs or Cs2Te, selected for low work functions or favorable quantum efficiency. Photon energies span the ultraviolet to x-ray range, depending on the material and desired emission.

History and impact. The phenomenon was explained by Albert Einstein in 1905, linking photon energy to electron emission and winning the theory a decisive place in quantum physics. Today, photoemission techniques are fundamental in surface science and condensed matter physics, and photocathodes remain essential components in scientific instruments and certain imaging devices.