Editing Photoelectric effect (section)

===Experimental observation of photoelectric emission===
Even though photoemission can occur from any material, it is most readily observed from metals and other conductors. This is because the process produces a charge imbalance which, if not neutralized by current flow, results in the increasing potential barrier until the emission completely ceases. The energy barrier to photoemission is usually increased by nonconductive oxide layers on metal surfaces, so most practical experiments and devices based on the photoelectric effect use clean metal surfaces in evacuated tubes. Vacuum also helps observing the electrons since it prevents gases from impeding their flow between the electrodes.{{citation needed|date=November 2023}}

Sunlight is an inconsistent and variable source of ultraviolet light. Cloud cover, ozone concentration, altitude, and surface reflection all alter the amount of UV. Laboratory sources of UV are based on [[xenon arc lamps]] or, for more uniform but weaker light, [[fluorescent lamps]].<ref>{{Cite journal |last=Diffey |first=Brian L. |date=2002-09-01 |title=Sources and measurement of ultraviolet radiation |url=https://linkinghub.elsevier.com/retrieve/pii/S1046202302002049 |journal=Methods |volume=28 |issue=1 |pages=4–13 |doi=10.1016/S1046-2023(02)00204-9 |pmid=12231182 |issn=1046-2023}}</ref>
More specialized sources include [[Laser|ultraviolet lasers]]<ref>{{Cite journal |last=Savage |first=Neil |date=February 2007 |title=Ultraviolet lasers |url=https://www.nature.com/articles/nphoton.2006.95 |journal=Nature Photonics |language=en |volume=1 |issue=2 |pages=83–85 |doi=10.1038/nphoton.2006.95 |bibcode=2007NaPho...1...83S |issn=1749-4893|url-access=subscription }}</ref> and [[synchrotron radiation]].<ref>{{Cite journal |last1=Ederer |first1=D.L. |last2=Saloman |first2=E.B. |last3=Ebner |first3=S.C. |last4=Madden |first4=R.P. |date=November 1975 |title=The use of synchrotron radiation as an absolute source of VUV radiation |url=https://nvlpubs.nist.gov/nistpubs/jres/79A/jresv79An6p761_A1b.pdf |journal=Journal of Research of the National Bureau of Standards Section A: Physics and Chemistry |language=en |volume=79A |issue=6 |pages=761–774 |doi=10.6028/jres.079A.032 |issn=0022-4332 |pmc=6589412 |pmid=32184529}}</ref>

[[File: Photoelectric effect measurement apparatus - microscopic picture.svg|thumb|Schematic of the experiment to demonstrate the photoelectric effect. Filtered, monochromatic light of a certain wavelength strikes the emitting electrode (E) inside a vacuum tube. The collector electrode (C) is biased to a voltage V<sub>C</sub> that can be set to attract the emitted electrons, when positive, or prevent any of them from reaching the collector when negative.|alt=]]
The classical setup to observe the photoelectric effect includes a light source, a set of filters to [[Monochromator|monochromatize]] the light, a [[vacuum tube]] transparent to ultraviolet light, an emitting electrode (E) exposed to the light, and a collector (C) whose voltage ''V''<sub>C</sub> can be externally controlled.{{citation needed|date=November 2023}}

A positive external voltage is used to direct the photoemitted electrons onto the collector. If the frequency and the intensity of the incident radiation are fixed, the photoelectric current ''I'' increases with an increase in the positive voltage, as more and more electrons are directed onto the electrode. When no additional photoelectrons can be collected, the photoelectric current attains a saturation value. This current can only increase with the increase of the intensity of light.{{citation needed|date=November 2023}}

An increasing negative voltage prevents all but the highest-energy electrons from reaching the collector. When no current is observed through the tube, the negative voltage has reached the value that is high enough to slow down and stop the most energetic photoelectrons of kinetic energy ''K''<sub>max</sub>. This value of the retarding voltage is called the ''stopping potential'' or ''cut off'' potential ''V''<sub>o</sub>.<ref>{{cite book|last1=Gautreau|first1=R.|title=Schaum's Outline of Modern Physics|last2=Savin|first2=W.|publisher=[[McGraw-Hill]]|year=1999|isbn=0-07-024830-3|edition=2nd|pages=60–61}}</ref> Since the work done by the retarding potential in stopping the electron of charge ''e'' is ''eV''<sub>o</sub>, the following must hold ''eV''<sub>o</sub>&nbsp;=&nbsp;''K''<sub>max.</sub>

The current-voltage curve is sigmoidal, but its exact shape depends on the experimental geometry and the electrode material properties.

For a given metal surface, there exists a certain minimum frequency of incident [[radiation]] below which no photoelectrons are emitted. This frequency is called the ''threshold frequency''. Increasing the frequency of the incident beam increases the maximum kinetic energy of the emitted photoelectrons, and the stopping voltage has to increase. The number of emitted electrons may also change because the [[probability]] that each photon results in an emitted electron is a function of photon energy{{Citation needed|reason=give a more precise reason for this probabilist effect, other than frequency|date=May 2024}}.

An increase in the intensity of the same monochromatic light (so long as the intensity is not too high<ref name=" Zhang1996">{{cite journal|last1=Zhang|first1=Q.|year=1996|title=Intensity dependence of the photoelectric effect induced by a circularly polarized laser beam|journal=[[Physics Letters A]]|volume=216|issue=1–5|page=125|bibcode=1996PhLA..216..125Z|doi=10.1016/0375-9601(96)00259-9}}</ref>), which is proportional to the number of photons impinging on the surface in a given time, increases the rate at which electrons are ejected—the photoelectric current ''I—''but the kinetic energy of the photoelectrons and the stopping voltage remain the same. For a given metal and frequency of incident radiation, the rate at which photoelectrons are ejected is directly proportional to the intensity of the incident light.

The time lag between the incidence of radiation and the emission of a photoelectron is very small, less than 10<sup>−9</sup> second. Angular distribution of the photoelectrons is highly dependent on [[polarization (waves)|polarization]] (the direction of the electric field) of the incident light, as well as the emitting material's quantum properties such as atomic and molecular orbital symmetries and the [[electronic band structure]] of crystalline solids. In materials without macroscopic order, the distribution of electrons tends to peak in the direction of polarization of linearly polarized light.<ref name="Bupp">{{cite journal
 |last1=Bubb |first1=F.
 |year=1924
 |title=Direction of Ejection of Photo-Electrons by Polarized X-rays
 |journal=[[Physical Review]]
 |volume=23|issue=2|pages=137–143
 |bibcode=1924PhRv...23..137B
 |doi=10.1103/PhysRev.23.137
}}</ref> The experimental technique that can measure these distributions to infer the material's properties is [[angle-resolved photoemission spectroscopy]].