Photo effect

The photoelectric effect or photoelectric effect is the phenomenon of the interaction of light or any other electromagnetic radiation with a substance, in which the energy of photons is transmitted to the electrons of a substance. In condensed (solid and liquid) substances, they emit an external (absorption of photons is accompanied by the departure of electrons outside the body) and internal (electrons, remaining in the body, change their energy state in it) the photo effect. The photoelectric effect in gases consists in the ionization of atoms or molecules under the action of radiation ^[1] .

The laws of the external photo effect :

The wording of the 1st law of the photoelectric effect (Stoletov's law) : The strength of the saturation photocurrent is directly proportional to the intensity of light emission ^[2] .

According to the 2nd law of the photoelectric effect , the maximum kinetic energy of electrons erupted by light increases with the frequency of light and does not depend on its intensity ^[2] .

The third law of the photoelectric effect : for each substance at a certain state of its surface there is a limiting frequency of light, below which the photoelectric effect is not observed. This frequency and the corresponding wavelength is called the red border of the photoelectric effect ^[2] .

In addition, the photoelectric effect has the property of practical inertness . It occurs immediately when the surface of the body is illuminated, provided that the frequency of the light is higher or equal to the red border of the photoelectric effect and the effect exists ^[2] .

A theoretical explanation of these laws was given in 1905 by Einstein . According to him, electromagnetic radiation is a stream of individual quanta ( photons ) with energy h ν each, where h is the Planck constant . With the photoelectric effect, a part of the incident electromagnetic radiation from the metal surface is reflected, and a part penetrates into the surface layer of the metal and is absorbed there. Having absorbed a photon, the electron receives energy from it and, making work exit A , leaves the metal: $h\nu =A+W_{k},$ ${\ displaystyle h \ nu = A + W_ {k},}$ $h \ nu = A + W _ {{k}},$ Where $W_{k}$ ${\ displaystyle W_ {k}}$ $W _ {{k}}$ - the maximum kinetic energy that an electron has when it leaves the metal.

Content

Opening History

In 1839, Alexander Becquerel observed ^{[3] the} photovoltaic effect in the electrolyte.

In 1873, Willoughby Smith discovered that selenium is photoconductive ^[4] ^[5] .

The external photoelectric effect was discovered in 1887 by Heinrich Hertz ^[6] ^[7] ^[8] . When working with an open resonator, he noticed that if we shine ultraviolet light on zinc arresters, the passage of the spark is much easier.

In 1888–90, the photoelectric effect was systematically studied by the Russian physicist Alexander Stoletov ^[9] , who published 6 papers ^[10] ^[11] ^[12] ^[13] ^[14] ^[15] . He made several important discoveries in this area, including the first law of the external photoelectric effect ^[16] .

More Stoletov came to the conclusion that "discharging action have, if not exclusively, then with tremendous superiority over other rays, the rays of the highest refractibility that are missing in the solar spectrum," that is, he came close to the conclusion about the existence of the red border of the photo effect. In 1891, Elster and Geitel, when studying alkali metals, came to the conclusion that the higher the electropositivity of the metal, the lower the frequency limit at which it becomes photosensitive. ^[17]

Thomson in 1898 experimentally established that the flow of electric charge emerging from a metal with an external photoelectric effect is a stream of particles discovered by it earlier (later called electrons). Therefore, an increase in the photocurrent with increasing illumination should be understood as an increase in the number of electrons knocked out with an increase in illumination.

Studies of the photoelectric effect by Philip Lenard in the years 1900-1902 showed that, contrary to classical electrodynamics , the energy of an outgoing electron is always strictly related to the frequency of the incident radiation and is almost independent of the intensity of irradiation .

The scheme of the experiment to study the photoelectric effect. A narrow frequency range is taken from the light and directed to the cathode inside the vacuum device. The voltage between the cathode and the anode sets an energy threshold between them. The current is judged on the achievement of the anode by electrons.

The photo effect was explained in 1905 by Albert Einstein (for which in 1921 , thanks to the nomination of the Swedish physicist Carl Wilhelm Oseen , he received the Nobel Prize ) based on the Max Planck conjecture about the quantum nature of light. Einstein’s work contained an important new hypothesis — if Planck in 1900 suggested that light was emitted only in quantized portions, then Einstein already believed that light existed only in the form of quantized portions. From the law of conservation of energy, in the representation of light in the form of particles ( photons ), follows the Einstein formula for the photoelectric effect:

h\nu =A+{\frac {mv^{2}}{2}}

{\ displaystyle h \ nu = A + {\ frac {mv ^ {2}} {2}}}

where A is the so-called. work function (minimum energy required to remove an electron from a substance), ${\frac {mv^{2}}{2}}$ ${\ displaystyle {\ frac {mv ^ {2}} {2}}}$ - the maximum kinetic energy of the outgoing electron, $\nu$ ${\ displaystyle \ nu}$ - frequency of the incident photon with energy $h\nu$ ${\ displaystyle h \ nu}$ , h is Planck's constant . This formula implies the existence of the red border of the photoelectric effect at T = 0 K, that is, the existence of the lowest frequency ( ${h\nu }_{\min }=A$ ${\ displaystyle {h \ nu} _ {\ min} = A}$ ), below which the photon energy is not enough to “knock out” an electron from a metal. The essence of the formula lies in the fact that the photon energy is spent on the ionization of the atom of matter and on the work necessary to "pull out" the electron, and the remainder goes into the kinetic energy of the electron.

In 1906-1915, Milliken studied the photoelectric effect. He was able to establish the exact dependence of the blocking voltage on the frequency (actually turned out to be linear) and, based on it, was able to calculate the Planck constant. “I spent ten years of my life testing this Einstein equation of 1905,” Milliken wrote, “and contrary to all my expectations I had to unconditionally admit in 1915 that it was experimentally confirmed, despite its absurdity, as it seemed that it contradicts everything that we know about the interference of light. " In 1923, Milliken was awarded the Nobel Prize in Physics "for his work on the definition of the elementary electric charge and photoelectric effect."

Photoeffect studies were among the very first quantum-mechanical studies.

External photo effect

External photoelectric effect ( photoemission ) is the emission of electrons by a substance under the action of electromagnetic radiation. Electrons emitted from a substance during an external photoelectric effect are called photoelectrons , and the electric current produced by them in an orderly movement in an external electric field is called a photocurrent .

A photocathode is an electrode of a vacuum electronic device, directly exposed to electromagnetic radiation and emitting electrons under the action of this radiation.

The dependence of spectral sensitivity on the frequency or wavelength of electromagnetic radiation is called the spectral characteristic of the photocathode.

The laws of the external photo effect

Stoletov's law: with a constant spectral composition of electromagnetic radiation incident on a photocathode, the saturation photocurrent is proportional to the cathode's energy illumination (otherwise: the number of photoelectrons knocked out of the cathode for 1 s is directly proportional to the radiation intensity):
$I_{n}~E_{e}$ ${\ displaystyle I_ {n} ~ E_ {e}}$ and $n_{\rm {cek}}~E_{e}$ ${\ displaystyle n _ {\ rm {cek}} ~ E_ {e}}$
The maximum initial speed of photoelectrons does not depend on the intensity of the incident light, but is determined only by its frequency.
For each substance, there is a red border of the photoelectric effect, that is, the minimum frequency of light $\nu _{0}$ {\ displaystyle \ nu _ {0}} (depending on the chemical nature of the substance and the state of the surface), below which the photoeffect is impossible.

Fowler's Theory

The basic laws of the external photoelectric effect for metals are well described by the Fowler theory ^[18] . According to it, after absorption of a photon in a metal, its energy passes to conduction electrons, as a result of which the electron gas in the metal consists of a mixture of gases with a normal Fermi-Dirac distribution and excited (shifted by $h\nu$ ${\ displaystyle h \ nu}$ ) distribution of energy. The photocurrent density is determined by the Fowler formula:

j={\begin{cases}{{B}_{1}}{{T}^{2}}\exp \left({\frac {h\nu -h{{\nu }_{\min }}}{kT}}\right),&h\nu \leqslant {{h\nu }_{\min }}-2kT,\\{{B}_{2}}{{T}^{2}}\left({\frac {{(h\nu -h{{\nu }_{\min }})}^{2}}{{{k}^{2}}{{T}^{2}}}}+{{B}_{3}}\right),&h\nu >{{h\nu }_{\min }+2kT},\\\end{cases}}

{\ displaystyle j = {\ begin {cases} {{B} _ {1}} {{T} ^ {2}} \ exp \ left ({\ frac {h \ nu -h {{\ nu} _ { \ min}}} {kT}} \ right), & h \ nu \ leqslant {{h \ nu} _ {\ min}} - 2kT, \\ {{B} _ {2}} {{T} ^ { 2}} \ left ({\ frac {{(h \ nu -h {{\ nu} _ {\ min}})} ^ {2}} {{{k} ^ {2}} {{T} ^ {2}}}} + {{B} _ {3}} \ right), & h \ nu> {{h \ nu} _ {\ min} + 2kT}, \\\ end {cases}}}

Where $B_{1}$ ${\ displaystyle B_ {1}}$ , $B_{2}$ ${\ displaystyle B_ {2}}$ , $B_{3}$ ${\ displaystyle B_ {3}}$ - constant coefficients depending on the properties of the irradiated metal. The formula is valid at photo-emission excitation energies not exceeding the value of the metal work function by more than a few electron volts. Fowler's theory is valid only in the case of light falling along the normal to the surface.

Quantum Output

An important quantitative characteristic of the photoelectric effect is the quantum yield Y, the number of electrons emitted per one photon falling on the surface of the body. The value of Y is determined by the properties of the substance, the state of its surface and the photon energy.

The quantum yield of the photoelectric effect from metals in the visible and near-UV regions Y <0.001 electron / photon. This is primarily due to the small depth of the output of photoelectrons, which is significantly less than the depth of absorption of light in the metal. Most photoelectrons dissipate their energy before approaching the surface and lose the opportunity to go into a vacuum. At a photon energy near the photoeffect threshold, most of the photoelectrons are excited below the vacuum level and do not contribute to the photoemission current. In addition, the reflection coefficient in the visible and near UV regions is large and only a small part of the radiation is absorbed in the metal. These restrictions are partially removed in the far UV region of the spectrum, where Y reaches a value of 0.01 electron / photon at a photon energy of E> 10 eV.

Internal Photo

The internal photoelectric effect is the phenomenon of increasing electrical conductivity and decreasing resistance caused by irradiation ^[19] . It is explained by the redistribution of electrons over the energy states in solid and liquid semiconductors and dielectrics , which occurs under the action of radiation, manifests itself in a change in the concentration of charge carriers in the medium and leads to the appearance of photoconductivity or a valve photoelectric effect ^[20] .

Photoconductivity is the increase in the electrical conductivity of a substance under the action of radiation.

Gate photo effect

A valve photoelectric effect or a photoelectric effect in the barrier layer is a phenomenon in which photoelectrons leave the body, passing through the interface to another solid ( semiconductor ) or liquid ( electrolyte ).

Photovoltaic effect

The photovoltaic effect is the occurrence of an electromotive force under the action of electromagnetic radiation ^[21] . It is used to measure the intensity of the incident light (for example in photodiodes ) or to generate electricity in solar batteries .

Sensitized photo effect

A sensitized photoelectric effect is called a photoelectric effect, accompanied by a sensitization phenomenon , that is, a change in the magnitude and spectrum of photosensitivity in wide-gap photoconductors of organic and inorganic nature depending on the structure of molecular compounds ^[22] .

Photo piezoelectric effect

The photo piezoelectric effect is the phenomenon of the appearance in the semiconductor of a photo of an electromotive force under external non-uniform compression of the semiconductor ^[23] .

Photomagnetic effect

The photomagnetic effect is the appearance of an electromotive force in an illuminated homogeneous semiconductor in a magnetic field ^[23] .

Nuclear Photo

When a gamma-quantum is absorbed, the nucleus receives an excess of energy without changing its nucleon composition, and the nucleus with an excess of energy is a composite nucleus . Like other nuclear reactions , the absorption of a gamma-quantum by the nucleus is possible only with the fulfillment of the necessary energy and spin ratios. If the energy transferred to the nucleus exceeds the binding energy of the nucleon in the nucleus, then the decay of the resulting composite nucleus occurs most often with the emission of nucleons, mainly neutrons . Such decay leads to nuclear reactions. $(\gamma ,n)$ ${\ displaystyle (\ gamma, n)}$ and $(\gamma ,p)$ ${\ displaystyle (\ gamma, p)}$ , which are called photonuclear , and the phenomenon of nucleon emission (neutrons and protons ) in these reactions is called the nuclear photoelectric effect ^[24] .

Multi-photon Photo Effect

In a strong electromagnetic field with the atom in the elementary act of the photoelectric effect, several photons can interact. In this case, ionization of the atom is possible with the help of radiation with the energy of quanta $h\nu >{\frac {E_{n}}{n}}$ ${\ displaystyle h \ nu> {\ frac {E_ {n}} {n}}}$ . Six- and seven-photon ionization of inert gases has been registered ^[25] .

Current Research

Experiments at the National Metrology Institute of Germany Physikalisch-Technische Bundesanstalt , the results of which were published on April 24, 2009 in the Physical Review Letters ^[26] , in a soft X-ray wavelength range at a power density of several pet watts (10 ¹⁵ W) per square centimeter The generally accepted theoretical model of the photoeffect may be incorrect.

Comparative quantitative studies of various materials have shown that the depth of interaction between radiation and matter substantially depends on the structure of the atoms of this substance and the correlation between the internal electron shells. In the case of c xenon , which was used in experiments, the effect of a photon packet in a short pulse leads, apparently, to the simultaneous emission of many electrons from the inner shells ^[27] .

Notes

↑ Yavorsky B. M. , Detlaf A. A. , Lebedev A. K. Handbook of physics for engineers and university students. - M., Onyx, 2007. - Circulation 5100 copies. - ISBN 978-5-488-01248-6 . - with. 725
↑ ¹ ² ³ ⁴ Yavorsky B. М. , Pinsky A. А. Osnovy fiziki. Volume 2. - M., Science , 1974. - Circulation 169000 copies. - with. 197
↑ AE Becquerel (1839). “Mémoire sur les effets électriques produits sous l'influence des rayons solaires”. Comptes Rendus 9: 561-567
↑ Smith, W. (1873). "Effect of Light on Selenium during the passage of an Electric Current". Nature 7 (173): 303. Bibcode: 1873Natur ... 7R.303 .. doi: 10.1038 / 007303e0
↑ TSB, PHOTOCONDUCTIVITY
↑ http://www.britannica.com/science/photoelectric-effect "Heinrich Rudolf Hertz."
↑ H. Hertz (1887), Ueber einen Einfluss des ultravioletten Lichtes auf die electrische Entladung (An effect of ultraviolet light on electrical discharge) / Ann. Phys. , 267: 983-1000. doi: 10.1002 / andp.18872670827 (him)
↑ Stig Lundqvist, Physics, 1901–1921 // World Scientific, 1998, ISBN 9789810234010 , p.121 (eng.)
↑ TSB, PHOTOEFFECT
↑ Stoletow, A. Sur une sorte de courants electriques provoques par les rayons ultraviolets (French) // Comptes Rendus : magazine. - 1888. - Vol. CVI . - P. 1149 . (Reprinted in Stoletow, MA, of current-produced ultra-violet rays (English) // Philosophical Magazine Series 5: journal. - 1888. - Vol. 26 , no. 160. - P. 317. - DOI : 10.1080 / 14786448808628270 ; abstract in Beibl. Ann. D. Phys. 12, 605, 1888).
↑ Stoletow, A. Sur les courants actino-electriques au travers deTair (fr.) // Comptes Rendus : magazine. - 1888. - Vol. CVI . - P. 1593 . (Abstract in Beibl. Ann. D. Phys. 12, 723, 1888).
↑ Stoletow, A. Suite des recherches actino-électriques (Neopr.) // Comptes Rendus . - 1888. - T. CVII . - p . 91 . (Abstract in Beibl. Ann. D. Phys. 12, 723, 1888).
↑ Stoletow, A. Sur les phénomènes actino-électriques (Neop.) // Comptes Rendus . - 1889. - T. CVIII . - p . 1241 .
↑ Stoletov, A. Actin-electric research (Rus.) // Journal of the Russian Physico-Chemical Society. - 1889. - V. 21 . - p . 159 .
↑ Stoletow, A. Sur les courants actino-électriques dans l'air raréfié (fr.) // Journal de Physique : magazine. - 1890. - Vol. 9 - P. 468 . - DOI : 10.1051 / jphystap: 018900090046800 .
↑ TSB, STOLETOV ALEXANDER GRIGORIEVICH
↑ Dukov V. M. Historical reviews in the course of high school physics. M., Enlightenment 1983 160 p.
↑ L.N. Dobretsov, M.V. Gomoyunova. Emission electronics . - M .: Science, 1966. - P. 564. (inaccessible link)
↑ Yavorsky B. М. , Pinsky A. А. Fundamentals of Physics. Volume 2. - M., Science , 1974. - Circulation 169000 copies. - with. 336
↑ Kireev, PS Physics of semiconductors. - M., High School , 1975. - Circulation 30000 copies. - with. 537-546
↑ Photovoltaic effect - an article from the Great Soviet Encyclopedia .
↑ Akimov, IA, Cherkasov, Yu.A., Cherkashin, MI Sensitized photo effect. - M .: Science, 1980. - p. 384.
↑ ¹ ² Tauts J. Photo and thermoelectric phenomena in semiconductors. - M .: IL, 1962. - p. 141.
↑ A. N. Klimov. Nuclear physics and nuclear reactors. - M .: Energoatomizdat, 1985. - p. 352.
↑ Quantum electronics. Little encyclopedia. - M .: Soviet Encyclopedia, 1969. - P. 431.
↑ Phys. Rev. Lett. 102, 163002 (2009): Extreme Ultraviolet Laser Excites Atomic Giant Resonance
↑ Discovered limitations of the classic photoelectric effect for x-rays | Nanotechnology Nanonewsnet

Links

Photo effect // Great Soviet Encyclopedia : [in 30 t.] / Ch. ed. A. M. Prokhorov . - 3rd ed. - M .: Soviet Encyclopedia, 1969-1978.
Photo effect - an article from the Physical Encyclopedia

[1] Yavorsky B. M. , Detlaf A. A. , Lebedev A. K. Handbook of physics for engineers and university students. - M., Onyx, 2007. - Circulation 5100 copies. - ISBN 978-5-488-01248-6 . - with. 725

[Javor-2] ¹ ² ³ ⁴ Yavorsky B. М. , Pinsky A. А. Osnovy fiziki. Volume 2. - M., Science , 1974. - Circulation 169000 copies. - with. 197

[3] AE Becquerel (1839). “Mémoire sur les effets électriques produits sous l'influence des rayons solaires”. Comptes Rendus 9: 561-567

[4] Smith, W. (1873). "Effect of Light on Selenium during the passage of an Electric Current". Nature 7 (173): 303. Bibcode: 1873Natur ... 7R.303 .. doi: 10.1038 / 007303e0

[5] TSB, PHOTOCONDUCTIVITY

[6] ttp://www.britannica.com/science/photoelectric-effect "Heinrich Rudolf Hertz."

[7] H. Hertz (1887), Ueber einen Einfluss des ultravioletten Lichtes auf die electrische Entladung (An effect of ultraviolet light on electrical discharge) / Ann. Phys. , 267: 983-1000. doi: 10.1002 / andp.18872670827 (him)

[8] Stig Lundqvist, Physics, 1901–1921 // World Scientific, 1998, ISBN 9789810234010 , p.121 (eng.)

[9] TSB, PHOTOEFFECT

[10] Stoletow, A. Sur une sorte de courants electriques provoques par les rayons ultraviolets (French) // Comptes Rendus : magazine. - 1888. - Vol. CVI . - P. 1149 . (Reprinted in Stoletow, MA, of current-produced ultra-violet rays (English) // Philosophical Magazine Series 5: journal. - 1888. - Vol. 26 , no. 160. - P. 317. - DOI : 10.1080 / 14786448808628270 ; abstract in Beibl. Ann. D. Phys. 12, 605, 1888).

[11] Stoletow, A. Sur les courants actino-electriques au travers deTair (fr.) // Comptes Rendus : magazine. - 1888. - Vol. CVI . - P. 1593 . (Abstract in Beibl. Ann. D. Phys. 12, 723, 1888).

[12] Stoletow, A. Suite des recherches actino-électriques (Neopr.) // Comptes Rendus . - 1888. - T. CVII . - p . 91 . (Abstract in Beibl. Ann. D. Phys. 12, 723, 1888).

[13] Stoletow, A. Sur les phénomènes actino-électriques (Neop.) // Comptes Rendus . - 1889. - T. CVIII . - p . 1241 .

[14] Stoletov, A. Actin-electric research (Rus.) // Journal of the Russian Physico-Chemical Society. - 1889. - V. 21 . - p . 159 .

[15] Stoletow, A. Sur les courants actino-électriques dans l'air raréfié (fr.) // Journal de Physique : magazine. - 1890. - Vol. 9 - P. 468 . - DOI : 10.1051 / jphystap: 018900090046800 .

[16] TSB, STOLETOV ALEXANDER GRIGORIEVICH

[17] Dukov V. M. Historical reviews in the course of high school physics. M., Enlightenment 1983 160 p.

[18] L.N. Dobretsov, M.V. Gomoyunova. Emission electronics . - M .: Science, 1966. - P. 564. (inaccessible link)

[19] Yavorsky B. М. , Pinsky A. А. Fundamentals of Physics. Volume 2. - M., Science , 1974. - Circulation 169000 copies. - with. 336

[20] Kireev, PS Physics of semiconductors. - M., High School , 1975. - Circulation 30000 copies. - with. 537-546

[21] Photovoltaic effect - an article from the Great Soviet Encyclopedia .

[22] Akimov, IA, Cherkasov, Yu.A., Cherkashin, MI Sensitized photo effect. - M .: Science, 1980. - p. 384.

[Tauz-23] ¹ ² Tauts J. Photo and thermoelectric phenomena in semiconductors. - M .: IL, 1962. - p. 141.

[24] A. N. Klimov. Nuclear physics and nuclear reactors. - M .: Energoatomizdat, 1985. - p. 352.

[25] Quantum electronics. Little encyclopedia. - M .: Soviet Encyclopedia, 1969. - P. 431.

[26] Phys. Rev. Lett. 102, 163002 (2009): Extreme Ultraviolet Laser Excites Atomic Giant Resonance

[27] Discovered limitations of the classic photoelectric effect for x-rays | Nanotechnology Nanonewsnet