Photoemission

Photoemission

The ejection of electrons from a solid (or less commonly, a liquid) by incident electromagnetic radiation. Photoemission is also called the external photoelectric effect. The visible and ultraviolet regions of the electromagnetic spectrum are most often involved, although the infrared and x-ray regions are also of interest.

The salient experimental features of photoemission are the following: (1) There is no detectable time lag between irradiation of an emitter and the ejection of photoelectrons. (2) At a given frequency the number of photoelectrons ejected per second is proportional to the intensity of the incident radiation. (3) The photoelectrons have kinetic energies ranging from zero up to a well-defined maximum, which is proportional to the frequency of the incident radiation and independent of the intensity.

In 1905 Albert Einstein made the clarifying assumption that electromagnetic radiation had characteristics like those of particles when it delivered energy to electrons in the emitter. In Einstein's approach the light beam behaves like a stream of photons, each of energy hν, where h is Planck's constant, and ν is the frequency of the photon. The energy required to eject an electron from the emitter has a well-defined minimum value &phgr; called the photoelectric threshold energy. When a photon interacts with an electron, the latter absorbs the entire photon energy. See Photon

For hν values below the threshold, photoelectrons are not ejected. Even though the electrons absorb photon energy, they do not receive enough to surmount the potential barrier at the surface, which normally holds the electrons in the solid. For photon energies above &phgr;, the kinetic energies of photo-electrons range from zero up to a maximum value, E= hν - &phgr;. This is the Einstein photoelectric law, and E is commonly termed the Einstein maximum energy. See Heat radiation, Schottky effect