photoelectric effect


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photoelectric effect,

emission of electrons by substances, especially metals, when light falls on their surfaces. The effect was discovered by H. R. Hertz in 1887. The failure of the classical theory of electromagnetic radiation to explain it helped lead to the development of the quantum theoryquantum theory,
modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
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. According to classical theory, when light, thought to be composed of waves, strikes substances, the energy of the liberated electrons ought to be proportional to the intensity of light. Experiments showed that, although the electron current produced depends upon the intensity of the light, the maximum energy of the electrons was not dependent on the intensity. Moreover, classical theory predicted that the photoelectric current should not depend on the frequency of the light and that there should be a time lag between the reception of light on the surface and the emission of the electrons. Neither of these predictions was borne out by experiment. In 1905, Albert Einstein published a theory that successfully explained the photoelectric effect. It was closely related to Planck's theory of blackbodyblackbody,
in physics, an ideal black substance that absorbs all and reflects none of the radiant energy falling on it. Lampblack, or powdered carbon, which reflects less than 2% of the radiation falling on it, crudely approximates an ideal blackbody; a material consisting of a
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 radiation announced in 1900. According to Einstein's theory, the incident light is composed of discrete particles of energy, or quanta, called photons, the energy of each photonphoton
, the particle composing light and other forms of electromagnetic radiation, sometimes called light quantum. The photon has no charge and no mass. About the beginning of the 20th cent.
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 being proportional to its frequency according to the equation E=h&ugr;, where E is the energy, &ugr; is the frequency, and h is Planck's constant. Each photoelectron ejected is the result of the absorption of one photon. The maximum kinetic energy, KE, that any photoelectron can possess is given by KE = h&ugr;−W, where W is the work function, i.e., the energy required to free an electron from the material, varying with the particular material. The effect has a number of practical applications, most based on the photoelectric cellphotoelectric cell
or photocell,
device whose electrical characteristics (e.g., current, voltage, or resistance) vary when light is incident upon it. The most common type consists of two electrodes separated by a light-sensitive semiconductor material.
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.

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

photoelectric effect

[¦fōd·ō·i′lek·trik i‚fekt]
(electronics)
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