photoelectric effect
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photoelectric effect
photoelectric effect
(foh-toh-i-lek -trik) An effect whereby electrons are emitted by material exposed to electromagnetic radiation above a certain frequency. This frequency usually lies in the ultraviolet region of the spectrum for most solids, but can occur in the visible region. The number of ejected electrons depends on the intensity of the incident radiation. The electron velocity is proportional to the radiation frequency.Photoelectric Effect
any of various electrical phenomena that occur in substances when the substances are exposed to electromagnetic radiation.
A substance absorbs electromagnetic energy only in discrete portions called quanta, or photons; a quantum is equal to ħω, where ħ is Planck’s constant and ω is the frequency of the radiation. Photoelectric effects occur when the energy of an absorbed photon is expended on a quantum transition of an electron to a higher energy state. Depending on the relationship between the photon energy and the characteristic energies of a substance, such as the atomic and molecular excitation and ionization energies and the electronic work function, the absorption of electromagnetic radiation may give rise to various photoelectric effects.
If the photon energy is sufficient only to excite an atom, a change in the dielectric constant of a substance may occur; such a change is called the photodielectric effect. If the photon energy is sufficient to produce nonequilibrium charge carriers in a solid, that is, to produce conduction electrons and holes, the conductivity of the solid is changed (see). An electromotive force (emf) called a photo-emf (see) is generated in inhomogeneous solids, during nonuniform illumination, and in semiconductors placed in a magnetic field (seeKIKOIN-NOSKOV EFFECT). Examples of inhomogeneous solids include semiconductors with a nonuniform impurity distribution, particularly in the region of a p-n junction, near a junction between two dissimilar semiconductors (seeSEMICONDUCTOR HETEROJUNCTION), or near a metal-semiconductor junction. Photoconductivity and a photo-emf may also arise when photons are absorbed by conduction electrons, with the result that the mobility of the electrons is increased (seeMOBILITY OF CHARGE CARRIERS IN SOLIDS).
If ħω is large enough to ionize the atoms or molecules of a gas, photoionization occurs. When photon energy of such a magnitude is absorbed by the electrons of a liquid or solid, photoemission occurs if the electrons can reach the surface of the liquid or solid and, after overcoming the potential barrier at the surface, escape into a vacuum or some other medium. Photoemission is often referred to as the external photoelectric effect. In contrast to photoemission, any photoelectric effect that results from electronic transitions from bound to quasi-free states within a solid is called an internal photoelectric effect.
Photoelectric effects should be distinguished from the electrical phenomena that occur when solids are heated by electromagnetic radiation. All photoelectric effects are caused by the disruption of the equilibrium between a system of electrons, on the one hand, and atoms, molecules, or a crystal lattice, on the other hand. The nonequilibrium state of the electron system in a solid is maintained for a certain time after the absorption of a photon; during this time, a photoelectric effect may be observed. Afterward, the excess electron energy is dissipated—for example, it is transferred to the crystal lattice—and an equilibrium corresponding to a higher temperature is established in the solid. Photoelectric effects then vanish but, because of the heating of the solid, certain effects, which are called thermoelectric effects and are similar in their external characteristics to photoelectric effects, arise in the solid. Thermoelectric effects include the temperature dependence of conductivity, the pyroelectric effect (seePYROELECTRICITY), thermionic emission, and the generation of a thermoelectromotive force.
Semiconductors and dielectrics contain few conduction electrons. Therefore, even a small number of photons is sufficient to cause a substantial increase in the amount of electrons or in the electron energy. The heat capacity of a solid’s crystal lattice is very high in comparison with the heat capacity of the conduction-electron “gas.” Consequently, photoelectric effects occur in solids that are not very small when such solids absorb far less electromagnetic energy than is required to observe thermoelectric effects. The rise time of photoelectric effects is many times less than the rise time of thermoelectric effects and, unlike the latter, does not depend on the size of the solid or the quality of the thermal contact with other solids.
Because of the very high conductivity of metals, no internal photoelectric effects are observed in metals, and only photoemission occurs.
REFERENCES
Ryvkin, S. M. Fotoelektricheskie iavleniia v poluprovodnikakh. Moscow, 1963.Fotoelektronnye pribory. Moscow, 1965.
Pankove, J. Opticheskie protsessy v poluprovodnikakh. Moscow, 1973. (Translated from English.)
Sommer, A. Fotoemissionnye materialy. Moscow, 1973. (Translated from English.)
T. M. LIFSHITS