photoelectric effect

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.
..... Click the link for more information. . 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
..... Click the link for more information. 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.
..... Click the link for more information. 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.
..... Click the link for more information. .

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) photoelectricity

photoelectric effect

effect

[ĕ-fekt´] a result produced by an action.additive effect the combined effect produced by the action of two or more agents, being equal to the sum of their separate effects.adverse effect a symptom produced by a drug or therapy that is injurious to the patient.Bainbridge effect Bainbridge reflex.Bohr effect decreased affinity of hemoglobin for oxygen caused by an increase of carbon dioxide; the oxyhemoglobin dissociation curve is displaced to the right because of higher partial pressure of carbon dioxide and lower pH. See also Haldane effect.

The Bohr effect causing a shift to the right in the oxyhemoglobin dissociation curve.Crabtree effect the inhibition of oxygen consumption on the addition of glucose to tissues or microorganisms having a high rate of aerobic glycolysis; the converse of the Pasteur effect.cumulative effect the action of a drug or treatment resulting from repeated use.Doppler effect see doppler effect.experimenter e's demand characteristics.extrapyramidal e's the side effects caused by neuroleptic medications, including dystonias, parkinsonism, akathisia, and dyskinesia" >tardive dyskinesia.Haldane effect increased oxygenation of hemoglobin promotes dissociation of carbon dioxide; see also Bohr effect.Hawthorne effect a psychological response in which the subjects in a research study change their behavior simply because they are subjects in a study, not because of the research treatment.heel effect variation in x-ray beam intensity and projected focal spot size along the long axis of the x-ray tube from cathode to anode.parallax effect the position of the image on each emulsion of dual emulsion film; it is accentuated by tube-angled x-ray techniques.Pasteur effect the decrease in the rate of glycolysis and the suppression of lactate accumulation by tissues or microorganisms in the presence of oxygen.photoelectric effect ejection of electrons from matter as a result of interaction with photons from high frequency electromagnetic radiation, such as x-rays; the ejected electrons may be energetic enough to ionize multiple additional atoms.placebo effect the total of all nonspecific effects, both good and adverse, of treatment; it refers primarily to psychological and psychophysiological effects associated with the caregiver-patient relationship and the patient's expectations and apprehensions concerning the treatment. See also placebo.position effect in genetics, the changed effect produced by alteration of the relative positions of various genes on the chromosomes.pressure effect the sum of the changes that are due to obstruction of tissue drainage by pressure.proarrhythmic effect any new, more advanced form of arrhythmia caused by an antiarrhythmic agent, especially those that produce hemodynamically important symptoms. These arrhythmias occur less than 30 days after initiation of treatment and are not due to a new event such as acute myocardial infarction or hypokalemia.side effect a consequence other than that for which an agent is used, especially an adverse effect on another organ system.Somogyi effect see somogyi effect.

pho·to·e·lec·tric ef·fect

1. the loss of electrons from the surface of a metal on exposure to light; 2. a mode of interaction of radiation with matter in which all of the energy of the incident photon is absorbed, with ejection of a photoelectron and characteristic radiation from filling the vacancy from another shell; since the energy absorption per gram of tissue is proportional to the cube of the atomic number, this mode is important in diagnostic radiography.

photoelectric effect

Radiology That fractional ↓ in beam intensity of ionizing radiation due to photoelectric effect in a medium through which it passes. See Attenuation coefficient.

pho·to·e·lec·tric effect

(fōtō-ĕ-lektrik ĕ-fekt) The interaction of an incoming photon with the inner shell electron of an atom that causes the electron to be ejected, resulting in a photoelectron.

单词	photoelectric effect
释义	photoelectric effect photoelectric effect n. Ejection of electrons from a substance by incident electromagnetic radiation, especially by visible light. photoelectric effect n 1. (General Physics) the ejection of electrons from a solid by an incident beam of sufficiently energetic electromagnetic radiation 2. (General Physics) any phenomenon involving electricity and electromagnetic radiation, such as photoemission photoelectric effect 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. ..... Click the link for more information. . 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 ..... Click the link for more information. 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. ..... Click the link for more information. 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. ..... Click the link for more information. . 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) photoelectricity photoelectric effect effect [ĕ-fekt´] a result produced by an action.additive effect the combined effect produced by the action of two or more agents, being equal to the sum of their separate effects.adverse effect a symptom produced by a drug or therapy that is injurious to the patient.Bainbridge effect Bainbridge reflex.Bohr effect decreased affinity of hemoglobin for oxygen caused by an increase of carbon dioxide; the oxyhemoglobin dissociation curve is displaced to the right because of higher partial pressure of carbon dioxide and lower pH. See also Haldane effect.The Bohr effect causing a shift to the right in the oxyhemoglobin dissociation curve.Crabtree effect the inhibition of oxygen consumption on the addition of glucose to tissues or microorganisms having a high rate of aerobic glycolysis; the converse of the Pasteur effect.cumulative effect the action of a drug or treatment resulting from repeated use.Doppler effect see doppler effect.experimenter e's demand characteristics.extrapyramidal e's the side effects caused by neuroleptic medications, including dystonias, parkinsonism, akathisia, and dyskinesia" >tardive dyskinesia.Haldane effect increased oxygenation of hemoglobin promotes dissociation of carbon dioxide; see also Bohr effect.Hawthorne effect a psychological response in which the subjects in a research study change their behavior simply because they are subjects in a study, not because of the research treatment.heel effect variation in x-ray beam intensity and projected focal spot size along the long axis of the x-ray tube from cathode to anode.parallax effect the position of the image on each emulsion of dual emulsion film; it is accentuated by tube-angled x-ray techniques.Pasteur effect the decrease in the rate of glycolysis and the suppression of lactate accumulation by tissues or microorganisms in the presence of oxygen.photoelectric effect ejection of electrons from matter as a result of interaction with photons from high frequency electromagnetic radiation, such as x-rays; the ejected electrons may be energetic enough to ionize multiple additional atoms.placebo effect the total of all nonspecific effects, both good and adverse, of treatment; it refers primarily to psychological and psychophysiological effects associated with the caregiver-patient relationship and the patient's expectations and apprehensions concerning the treatment. See also placebo.position effect in genetics, the changed effect produced by alteration of the relative positions of various genes on the chromosomes.pressure effect the sum of the changes that are due to obstruction of tissue drainage by pressure.proarrhythmic effect any new, more advanced form of arrhythmia caused by an antiarrhythmic agent, especially those that produce hemodynamically important symptoms. These arrhythmias occur less than 30 days after initiation of treatment and are not due to a new event such as acute myocardial infarction or hypokalemia.side effect a consequence other than that for which an agent is used, especially an adverse effect on another organ system.Somogyi effect see somogyi effect. pho·to·e·lec·tric ef·fect 1. the loss of electrons from the surface of a metal on exposure to light; 2. a mode of interaction of radiation with matter in which all of the energy of the incident photon is absorbed, with ejection of a photoelectron and characteristic radiation from filling the vacancy from another shell; since the energy absorption per gram of tissue is proportional to the cube of the atomic number, this mode is important in diagnostic radiography. photoelectric effect Radiology That fractional ↓ in beam intensity of ionizing radiation due to photoelectric effect in a medium through which it passes. See Attenuation coefficient. pho·to·e·lec·tric effect (fōtō-ĕ-lektrik ĕ-fekt) The interaction of an incoming photon with the inner shell electron of an atom that causes the electron to be ejected, resulting in a photoelectron. LegalSeeEffectSee PEE See PEEThesaurusSeeeffect
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