Editing Photoelectric effect (section)

==History==
===19th century===
In 1839, [[A. E. Becquerel|Alexandre Edmond Becquerel]] discovered the related [[photovoltaic effect]] while studying the effect of light on [[electrolytic cell]]s.<ref name="Petrova-KochHezel2009">{{cite book|author1=Vesselinka Petrova-Koch|author2=Rudolf Hezel|author3=Adolf Goetzberger|chapter=Milestones of Solar Conversion and Photovoltaics |series=Springer Series in Optical Sciences |volume=140 |title=High-Efficient Low-Cost Photovoltaics: Recent Developments|year=2009|publisher=Springer|isbn=978-3-540-79358-8|pages=1–|doi=10.1007/978-3-540-79359-5_1|s2cid=108793685 }}</ref> Though not equivalent to the photoelectric effect, his work on [[photovoltaics]] was instrumental in showing a strong relationship between light and electronic properties of materials. In 1873, [[Willoughby Smith]] discovered [[photoconductivity]] in [[selenium]] while testing the metal for its high resistance properties in conjunction with his work involving submarine telegraph cables.<ref name="Smith1873">{{cite journal|author=Smith, W.|url=https://www.histv.net/willoughby-smith|year=1873|title=Effect of Light on Selenium during the passage of an Electric Current|doi=10.1038/007303e0|journal=Nature|page=303|volume=7|issue=173|bibcode = 1873Natur...7R.303. |doi-access=free}}</ref>

[[Julius Elster|Johann Elster]] (1854–1920) and [[Hans Friedrich Geitel|Hans Geitel]] (1855–1923), students in [[Heidelberg]], investigated the effects produced by light on electrified bodies and developed the first practical photoelectric cells that could be used to measure the intensity of light.<ref name="Ref_e">Asimov, A. (1964) ''[[Asimov's Biographical Encyclopedia of Science and Technology]]'', Doubleday, {{ISBN|0-385-04693-6}}.</ref><ref name="BudWarner1998">{{cite book|author1=Robert Bud|author2=Deborah Jean Warner|title=Instruments of Science: An Historical Encyclopedia|year=1998|publisher=Science Museum, London, and National Museum of American History, Smithsonian Institution|isbn=978-0-8153-1561-2}}</ref>{{rp|458}} They arranged metals with respect to their power of discharging negative electricity: [[rubidium]], [[potassium]], [[alloy]] of potassium and sodium, [[sodium]], [[lithium]], [[magnesium]], [[thallium]] and [[zinc]]; for [[copper]], [[platinum]], [[lead]], [[iron]], [[cadmium]], [[carbon]], and [[Mercury (element)|mercury]] the effects with ordinary light were too small to be measurable. The order of the metals for this effect was the same as in Volta's series for contact-electricity, the most electropositive metals giving the largest photo-electric effect.
[[File:Gold leaf electroscope 1869.png|alt=|thumb|282x282px|Gold leaf [[electroscope]] demonstrating the photoelectric effect. When the electroscope disk is negatively charged with excess electrons, the gold leaves mutually repel. If high-energy light (such as ultraviolet) is then shone on the disk, electrons are emitted by the photoelectric effect and the leaf repulsion ceases. But if the light used has insufficient energy to stimulate electron emission, the leaves stay separated regardless of duration.]]
In 1887, [[Heinrich Hertz]] observed the photoelectric effect<ref name="Ref_f">
{{cite journal|last=Hertz|first=Heinrich|date=1887|title=Ueber einen Einfluss des ultravioletten Lichtes auf die electrische Entladung|url=https://zenodo.org/record/1423827|journal=[[Annalen der Physik]]|volume=267|issue=8|pages=983–1000|bibcode=1887AnP...267..983H|doi=10.1002/andp.18872670827}}
</ref> and reported on the production and reception<ref>{{Cite journal|last=Hertz|first=H.|date=1887|title=Ueber sehr schnelle electrische Schwingungen|journal=Annalen der Physik und Chemie|language=en|volume=267|issue=7|pages=421–448|doi=10.1002/andp.18872670707|bibcode=1887AnP...267..421H|issn=0003-3804|url=https://zenodo.org/record/1423823}}</ref> of electromagnetic waves.<ref name="Smithsonian report">{{cite book |last1=Bloch |first1=Eugene |title=Annual Report Of The Board Of Regents Of The Smithsonian Institution 1913 |date=1914 |publisher=Smithsonian Institution |location=Washington, DC |page=239 |chapter-url=https://archive.org/details/in.ernet.dli.2015.104291/page/n283/mode/2up |access-date=2 May 2020 |chapter=Recent developments in electromagnetism}}</ref> The receiver in his apparatus consisted of a coil with a [[spark gap]], where a spark would be seen upon detection of electromagnetic waves. He placed the apparatus in a darkened box to see the spark better. However, he noticed that the maximum spark length was reduced when inside the box. A glass panel placed between the source of electromagnetic waves and the receiver absorbed ultraviolet radiation that assisted the electrons in jumping across the gap. When removed, the spark length would increase. He observed no decrease in spark length when he replaced the glass with quartz, as [[quartz]] does not absorb UV radiation.{{citation needed|date=November 2023}}

The discoveries by Hertz led to a series of investigations by [[Wilhelm Hallwachs]],<ref>{{Cite journal|last=Hallwachs|first=Wilhelm|date=1888|title=Ueber den Einfluss des Lichtes auf electrostatisch geladene Körper|journal=Annalen der Physik|language=en|volume=269|issue=2|pages=301–312|doi=10.1002/andp.18882690206|bibcode=1888AnP...269..301H|issn=1521-3889|url=https://zenodo.org/record/1423835}}</ref><ref name="Ref_g">Hallwachs, Wied. Ann. xxxiii. p. 301, 1888.</ref> Hoor,<ref name="Ref_h">Hoor, Repertorium des Physik, xxv. p. 91, 1889.</ref> [[Augusto Righi]]<ref name="Ref_i">Bighi, C. R. cvi. p. 1349; cvii. p. 559, 1888</ref> and [[Aleksandr Stoletov|Aleksander Stoletov]]<ref name="Ref_j">Stoletov. C. R. cvi. pp. 1149, 1593; cvii. p. 91; cviii. p. 1241; Physikalische Revue, Bd. i., 1892.</ref><ref name="Stoletov">
* {{cite journal |author=Stoletov, A. |year=1888 |title=Sur une sorte de courants electriques provoques par les rayons ultraviolets |journal=[[Comptes rendus de l'Académie des sciences|Comptes Rendus]] |volume=CVI |page=1149}} (Reprinted in {{cite journal|doi=10.1080/14786448808628270|title=On a kind of electric current produced by ultra-violet rays|year=1888|last1=Stoletov|first1=M.A.|journal=Philosophical Magazine |series=Series 5|volume=26|issue=160|page=317|url=https://zenodo.org/record/1431191}}; abstract in Beibl. Ann. d. Phys. 12, 605, 1888).
* {{cite journal |author=Stoletov, A. |year=1888 |title=Sur les courants actino-electriqies au travers deTair |journal=[[Comptes rendus de l'Académie des sciences|Comptes Rendus]] |volume=CVI |page=1593 }} (Abstract in Beibl. Ann. d. Phys. 12, 723, 1888).
* {{cite journal |author=Stoletov, A. |year=1888|title=Suite des recherches actino-electriques |journal=[[Comptes rendus de l'Académie des sciences|Comptes Rendus]] |volume=CVII |page=91}} (Abstract in Beibl. Ann. d. Phys. 12, 723, 1888).
* {{cite journal |author=Stoletov, A. |year=1889 |journal=[[Comptes rendus de l'Académie des sciences|Comptes Rendus]] |volume=CVIII |page=1241|title=Sur les phénomènes actino-électriques }}
* {{cite journal |author=Stoletov, A. |year=1889 |journal=Journal of the Russian Physico-chemical Society |volume=21 |page=159 |title=Актино-электрические исследовaния|language=ru}}
* {{cite journal |author=Stoletov, A. |year=1890 |journal=Journal de Physique |volume=9 |page=468|title=Sur les courants actino-électriques dans l'air raréfié|doi=10.1051/jphystap:018900090046800 |url=https://hal.archives-ouvertes.fr/jpa-00239138/document}}</ref> on the effect of light, and especially of ultraviolet light, on charged bodies. Hallwachs connected a zinc plate to an [[electroscope]]. He allowed ultraviolet light to fall on a freshly cleaned zinc plate and observed that the zinc plate became uncharged if initially negatively charged, positively charged if initially uncharged, and more positively charged if initially positively charged. From these observations he concluded that some negatively charged particles were emitted by the zinc plate when exposed to ultraviolet light. 

With regard to the ''Hertz effect'', the researchers from the start showed the complexity of the phenomenon of photoelectric fatigue—the progressive diminution of the effect observed upon fresh metallic surfaces. According to Hallwachs, [[ozone]] played an important part in the phenomenon,<ref name="Ref_m">{{cite journal|doi=10.1002/andp.19073280807|title=Über die lichtelektrische Ermüdung|year=1907|last1=Hallwachs|first1=W.|journal=Annalen der Physik|volume=328|issue=8|pages=459–516|bibcode = 1907AnP...328..459H |url=https://zenodo.org/record/1424105}}</ref> and the emission was influenced by oxidation, humidity, and the degree of polishing of the surface. It was at the time unclear whether fatigue is absent in a vacuum.{{citation needed|date=November 2023}}

In the period from 1888 until 1891, a detailed analysis of the photoeffect was performed by [[Aleksandr Stoletov]] with results reported in six publications.<ref name="Stoletov"/> Stoletov invented a new experimental setup which was more suitable for a quantitative analysis of the photoeffect. He discovered a direct proportionality between the intensity of light and the induced photoelectric current (the first law of photoeffect or [[Stoletov's law]]). He measured the dependence of the intensity of the photo electric current on the gas pressure, where he found the existence of an optimal gas pressure corresponding to a maximum [[photocurrent]]; this property was used for the creation of [[solar cell]]s.{{citation needed|date=October 2009}}

Many substances besides metals discharge negative electricity under the action of ultraviolet light. G. C. Schmidt<ref name="Ref_c">Schmidt, G. C. (1898) Wied. Ann. Uiv. p. 708.</ref> and O. Knoblauch<ref>{{Cite book|last=Knoblauch|first=O.|title=Zeitschrift für Physikalische Chemie|year=1899|volume=xxix|page=527}}</ref> compiled a list of these substances.

In 1897, [[J. J. Thomson]] investigated ultraviolet light in [[Geissler tube|Crookes tubes]].<ref name="Ref_o">''The International Year Book''. (1900). New York: Dodd, Mead & Company. p. 659.</ref> Thomson deduced that the ejected particles, which he called corpuscles, were of the same nature as [[cathode rays]]. These particles later became known as the electrons. Thomson enclosed a metal plate (a cathode) in a vacuum tube, and exposed it to high-frequency radiation.<ref>{{Cite book|title=Histories of the electron: the birth of microphysics|date=2001|publisher=MIT Press|author1=Buchwald, Jed Z. |author2=Warwick, Andrew.|isbn=978-0-262-26948-3|location=Cambridge, Mass.|oclc=62183406}}</ref> It was thought that the oscillating electromagnetic fields caused the atoms' field to resonate and, after reaching a certain amplitude, caused subatomic corpuscles to be emitted, and current to be detected. The amount of this current varied with the intensity and color of the radiation. Larger radiation intensity or frequency would produce more current.{{citation needed|date=October 2009}}

During the years 1886–1902, [[Wilhelm Hallwachs]] and [[Philipp Lenard]] investigated the phenomenon of photoelectric emission in detail. Lenard observed that a current flows through an evacuated glass tube enclosing two [[electrode]]s when ultraviolet radiation falls on one of them. As soon as ultraviolet radiation is stopped, the current also stops. This initiated the concept of photoelectric emission. The discovery of the ionization of gases by ultraviolet light was made by Philipp Lenard in 1900. As the effect was produced across several centimeters of air and yielded a greater number of positive ions than negative, it was natural to interpret the phenomenon, as J. J. Thomson did, as a ''Hertz effect'' upon the particles present in the gas.<ref name="Smithsonian report" />

===20th century===
In 1902, Lenard observed that the [[energy]] of individual emitted electrons was independent of the applied light intensity.<ref name="Ref_Lenard" /><ref>{{cite journal |title=Philipp Lenard and the Photoelectric Effect, 1889-1911 |first=Bruce R. |last=Wheaton |journal=Historical Studies in the Physical Sciences |volume=9 |year=1978 |pages=299–322 |doi=10.2307/27757381 |jstor=27757381}}</ref> This appeared to be at odds with Maxwell's [[wave theory of light]], which predicted that the electron energy would be proportional to the [[intensity (physics)|intensity]] of the radiation.

Lenard observed the variation in electron energy with light frequency using a powerful electric arc lamp which enabled him to investigate large changes in intensity. However, Lenard's results were qualitative rather than quantitative because of the difficulty in performing the experiments: the experiments needed to be done on freshly cut metal so that the pure metal was observed, but it oxidized in a matter of minutes even in the partial vacuums he used. The current emitted by the surface was determined by the light's intensity, or brightness: doubling the intensity of the light doubled the number of electrons emitted from the surface.{{citation needed|date=November 2023}}

Initial investigation of the photoelectric effect in gasses by Lenard<ref name="Ref_p">{{cite journal|doi=10.1051/radium:0190800508024001|title=L'ionisation de l'air par la lumière ultra-violette|year=1908|last1=Bloch|first1=E.|journal=Le Radium|volume=5|issue=8|page=240|url=https://hal.archives-ouvertes.fr/jpa-00242302/document}}</ref> were followed up by J. J. Thomson<ref name="Ref_q">{{cite journal|author=Thomson, J. J. |journal=Proc. Camb. Phil. Soc.|volume=14|page=417|year=1907|title=On the Ionisation of Gases by Ultra-Violet Light and on the evidence as to the Structure of Light afforded by its Electrical Effects}}</ref> and then more decisively by Frederic Palmer Jr.<ref name="Ref_r">{{cite journal|doi=10.1038/077582b0|title=Ionisation of Air by Ultra-violet Light|year=1908|last1=Palmer|first1=Frederic|journal=Nature|volume=77|issue=2008|page=582 |bibcode = 1908Natur..77..582P |s2cid=4028617|url=https://zenodo.org/record/1429499|doi-access=free}}</ref><ref>{{cite journal|doi=10.1103/PhysRevSeriesI.32.1|title=Volume Ionization Produced by Light of Extremely Short Wave-Length|year=1911|last1=Palmer|first1=Frederic|journal=Physical Review |series=Series I|volume=32|issue=1|pages=1–22|bibcode = 1911PhRvI..32....1P |url=https://zenodo.org/record/1525061}}</ref> The gas photoemission was studied and showed very different characteristics than those at first attributed to it by Lenard.<ref name="Smithsonian report" />

In 1900, while studying [[black-body radiation]], the German physicist [[Max Planck]] suggested in his "On the Law of Distribution of Energy in the Normal Spectrum"<ref>{{cite journal|last1=Planck|first1=Max|year=1901|title=Ueber das Gesetz der Energieverteilung im Normalspectrum (On the Law of Distribution of Energy in the Normal Spectrum)|journal=Annalen der Physik|volume=4|issue=3|page=553|bibcode=1901AnP...309..553P|doi=10.1002/andp.19013090310|doi-access=free}}</ref> paper that the energy carried by electromagnetic waves could only be released in ''packets'' of energy. In 1905, Albert Einstein published a paper advancing the hypothesis that light energy is carried in discrete quantized packets to explain experimental data from the photoelectric effect. Einstein theorized that the energy in each quantum of light was equal to the frequency of light multiplied by a constant, later called the [[Planck constant]]. A photon above a threshold frequency has the required energy to eject a single electron, creating the observed effect. This was a step in the development of [[quantum mechanics]]. In 1914, [[Robert Andrews Millikan|Robert A. Millikan]]'s highly accurate measurements of the Planck constant from the photoelectric effect supported Einstein's model, even though a corpuscular theory of light was for Millikan, at the time, "quite unthinkable".<ref>{{Cite journal|last=Holton|first=Gerald|date=1999-04-22|title=Centennial Focus: Millikan's Measurement of Planck's Constant|url=https://physics.aps.org/story/v3/st23|journal=Physics|language=en|volume=3|page=23 |doi=10.1103/physrevfocus.3.23}}</ref> Einstein was awarded the 1921 [[Nobel Prize in Physics]] for "his discovery of the law of the photoelectric effect",<ref name="Ref_s">{{cite web|title=The Nobel Prize in Physics 1921|url=http://nobelprize.org/nobel_prizes/physics/laureates/1921/index.html|access-date=2008-10-09|publisher=Nobel Foundation}}</ref> and Millikan was awarded the Nobel Prize in 1923 for "his work on the elementary charge of electricity and on the photoelectric effect".<ref>{{cite web|title=The Nobel Prize in Physics 1923|url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1923/press.html|access-date=2015-03-29|publisher=Nobel Foundation}}</ref> In quantum perturbation theory of atoms and solids acted upon by electromagnetic radiation, the photoelectric effect is still commonly analyzed in terms of waves; the two approaches are equivalent because photon or wave absorption can only happen between quantized energy levels whose energy difference is that of the energy of photon.<ref name="Lamb1968">{{cite news|first1 = Willis E. Jr.|last1 = Lamb|author-link = Willis Lamb|last2 = Scully|first2 = Marlan O.|title = The photoelectric effect without photons|location=Coral Gables, FL | institution = Center for Theoretical Physics, University of Miami|quote = we understand the photoeffect as being the result of a classical field falling on a quantized atomic electron |url = https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680009569.pdf|year = 1968}}</ref><ref name="Stefan2003" />

Albert Einstein's mathematical description of how the photoelectric effect was caused by absorption of [[quantum|quanta]] of light was in one of his [[Annus Mirabilis papers]], named "On a Heuristic Viewpoint Concerning the Production and Transformation of Light".<ref>Einstein, A. "[https://sites.pitt.edu/~jdnorton/lectures/Rotman_Summer_School_2013/Einstein_1905_docs/Einstein_Light_Quantum_WikiSource.pdf On a Heuristic Viewpoint Concerning the Emission and Transformation of Light]." Annalen der Physik 17 (1905)</ref> The paper proposed a simple description of ''energy quanta'', and showed how they explained the blackbody radiation spectrum. His explanation in terms of absorption of discrete quanta of light agreed with experimental results. It explained why the energy of photoelectrons was not dependent on incident light ''intensity''. This was a theoretical leap, but the concept was strongly resisted at first because it contradicted the wave theory of light that followed naturally from [[James Clerk Maxwell]]'s equations of electromagnetism, and more generally, the assumption of [[infinite divisibility]] of energy in physical systems.

Einstein's work predicted that the energy of individual ejected electrons increases linearly with the frequency of the light. The precise relationship had not at that time been tested. By 1905 it was known that the energy of photoelectrons increases with increasing ''frequency'' of incident light and is independent of the ''intensity'' of the light. However, the manner of the increase was not experimentally determined until 1914 when Millikan showed that Einstein's prediction was correct.<ref name="Ref_Millikan" />

The photoelectric effect helped to propel the then-emerging concept of wave–particle duality in the nature of light. Light simultaneously possesses the characteristics of both waves and particles, each being manifested according to the circumstances. The effect was impossible to understand in terms of the classical wave description of light,<ref name="Ref_u">Resnick, Robert (1972) ''Basic Concepts in Relativity and Early Quantum Theory'', Wiley, p. 137, {{ISBN|0-471-71702-9}}.</ref><ref name="Ref_">Knight, Randall D. (2004) ''Physics for Scientists and Engineers With Modern Physics: A Strategic Approach'', Pearson-Addison-Wesley, p. 1224, {{ISBN|0-8053-8685-8}}.</ref><ref name="Ref_v">Penrose, Roger (2005) ''The Road to Reality: A Complete Guide to the Laws of the Universe'', Knopf, p. 502, {{ISBN|0-679-45443-8}}</ref> as the energy of the emitted electrons did not depend on the intensity of the incident radiation. Classical theory predicted that the electrons would 'gather up' energy over a period of time, and then be emitted.<ref name="Ref_" /><ref name="Ref_w">Resnick, Robert (1972) ''Basic Concepts in Relativity and Early Quantum Theory'', Wiley, p. 138, {{ISBN|0-471-71702-9}}.</ref>
===21st century===
Research in recent years on the photoelectric effect has been focused on measurements on emission time of photoelectrons. For long, it was believed that photoemission is an instantaneous process. However, a seminal role in this field was played by experimental techniques on attosecond generation of pulses of light for studies on electron dynamics, which was recognised through the 2023 Nobel Prize in physics to Pierre Agostini, Ferenc Krausz and Anne L’Huillier.<ref name="NP">{{cite journal |title=Light as fast as electrons |journal=[[Nat. Phys.]] |volume=19 |year=2023 |issue=11 |pages=1520 |doi=10.1038/s41567-023-02305-y |bibcode=2023NatPh..19.1520. }}</ref> For example, in 2010, it was discovered that electron emission takes 20 attoseconds and that photoemission is associated with complex multielectron correlations and is not a single-electron process.<ref name="Schultze">{{cite journal |title=Delay in Photoemission , 1889-1911 |last1=Schultze |first1=M. |journal=[[Science (journal)|Science]] |volume=328 |year=2010 |issue=5986 |pages=1658–1662 |doi=10.1126/science.1189401|pmid=20576884 |url=https://mediatum.ub.tum.de/1579410 }}</ref> In a more recent work in the context of [[tungsten]], measurements on photoelectron emission indicated that around 100 [[attosecond]]s are required to liberate an electron.<ref name="Cavalieri">{{cite journal |title=Attosecond spectroscopy in condensed matter |last1=Cavalieri |first1=A. L. |last2=Müller |first2=N. |last3=Uphues |first3=Th. |last4=Yakovlev |first4=V. S. |last5=Baltuška |first5=A. |last6=Horvath |first6=B. |last7=Schmidt |first7=B. |last8=Blümel |first8=L. |last9=Holzwarth |first9=R. |last10=Hendel |first10=S. |last11=Drescher |first11=M. |last12=Kleineberg |first12=U. |last13=Echenique |first13=P. M. |last14=Kienberger |first14=R. |last15=Krausz |first15=F. |last16=Heinzmann |first16=U. |display-authors=5 |journal=[[Nature (journal)|Nature]] |volume=449 |year=2007 |issue=7165 |pages=1029–1032|doi=10.1038/nature06229|pmid=17960239 |bibcode=2007Natur.449.1029C }}</ref> In another research, the value was found to be 45 attoseconds.<ref name="Ossiander">{{cite journal |title=Absolute timing of the photoelectric effect, 1889-1911 |last1=Ossiander |first1=M. |last2=Riemensberger |first2=J. |last3=Neppl |first3=S. |last4=Mittermair |first4=M. |last5=Schäffer |first5=M. |last6=Duensing |first6=A. |last7=Wagner |first7=M. S. |last8=Heider |first8=R. |last9=Wurzer |first9=M. |last10=Gerl |first10=M. |last11=Schnitzenbaumer |first11=M. |last12=Barth |first12=J. V. |last13=Libisch |first13=F. |last14=Lemell |first14=C. |last15=Burgdörfer |first15=J. |last16=Feulner |first16=P. |last17=Kienberger |first17=R. |display-authors=5 |journal=[[Nature (journal)|Nature]] |volume=561 |year=2018 |issue=7723 |pages=374–377 |doi=10.1038/s41586-018-0503-6|pmid=30232421 |url=https://mediatum.ub.tum.de/1579367 }}</ref> A broad consensus is emerging towards the fact that photoemission is not instantaneous and involves finite time.

The role of electric field in photoelectric effect has also been empirically studied and it was found that electromagnetic radiation with a specific orientation of electric field can excite electrons leading to enhanced emission in the Terahertz range.<ref name="Ritchie">{{cite journal |title=An in-plane photoelectric effect in two-dimensional electron systems for terahertz detection |journal=[[Science Advances]] |volume=8 |year=2022|last1=Michailow |first1=W.|last2=Ritchie |first2=D.|issue=15 |pages=eabi8398 | doi=10.1126/sciadv.abi8398 |pmid=35427162 |pmc=9012455 |arxiv=2011.04177 |bibcode=2022SciA....8I8398M }}</ref>