According to classical electromagnetic theory, this effect can be attributed to the transfer of energy from the light to an electron. From this perspective, an alteration in the intensity of light would induce changes in the kinetic energy of the electrons emitted from the metal. Furthermore, according to this theory, a sufficiently dim light would be expected to show a time lag between the initial shining of its light and the subsequent emission of an electron. However, the experimental results did not correlate with either of the two predictions made by classical theory.
Instead, electrons are dislodged only by the impingement of photons when those photons reach or exceed a threshold frequency (energy). Below that threshold, no electrons are emitted from the material regardless of the light intensity or the length of time of exposure to the light. (Rarely, an electron will escape by absorbing two or more quanta. However, this is extremely rare because by the time it absorbs enough quanta to escape, the electron will probably have emitted the rest of the quanta.) To make sense of the fact that light can eject electrons even if its intensity is low, Albert Einstein proposed that a beam of light is not a wave propagating through space, but rather a collection of discrete wave packets (photons), each with energy hν. This shed light on Max Planck's previous discovery of the Planck relation (E = hν) linking energy (E) and frequency (ν) as arising from quantization of energy. The factor h is known as the Planck constant.
In 1887, Heinrich Hertz discovered that electrodes illuminated with ultraviolet light create electric sparks more easily. In 1900, while studying black-body radiation, the German physicist Max Planck suggested 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. This model contributed to the development of quantum mechanics. In 1914, Millikan's experiment supported Einstein's model of the photoelectric effect. Einstein was awarded the Nobel Prize in 1921 for "his discovery of the law of the photoelectric effect", and Robert Millikan was awarded the Nobel Prize in 1923 for "his work on the elementary charge of electricity and on the photoelectric effect".
The photoelectric effect requires photons with energies approaching zero (in the case of negative electron affinity) to over 1 MeV for core electrons in elements with a high atomic number. Emission of conduction electrons from typical metals usually requires a few electron-volts, corresponding to short-wavelength visible or ultraviolet light. Study of the photoelectric effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of wave–particle duality. Other phenomena where light affects the movement of electric charges include the photoconductive effect (also known as photoconductivity or photoresistivity), the photovoltaic effect, and the photoelectrochemical effect.
Photoemission can occur from any material, but it is most easily observable from metals or other conductors because the process produces a charge imbalance, and if this charge imbalance is not neutralized by current flow (enabled by conductivity), the potential barrier to emission increases until the emission current ceases. It is also usual to have the emitting surface in a vacuum, since gases impede the flow of photoelectrons and make them difficult to observe. Additionally, the energy barrier to photoemission is usually increased by thin oxide layers on metal surfaces if the metal has been exposed to oxygen, so most practical experiments and devices based on the photoelectric effect use clean metal surfaces in a vacuum.
When the photoelectron is emitted into a solid rather than into a vacuum, the term internal photoemission is often used, and emission into a vacuum distinguished as external photoemission.
Schematic
of experimental apparatus to demonstrate the photoelectric effect. The
filter passes light of certain wavelengths from the lamp at left. The
light strikes the curved electrode, and electrons are emitted. The
adjustable voltage can be increased until the current stops flowing.
This "stopping voltage" is a function only of the electrode material and
the frequency of the incident light, and is not affected by the
intensity of the light.
Emission mechanism
The photons of a light beam have a characteristic energy proportional to the frequency of the light.
In the photoemission process, if an electron within some material
absorbs the energy of one photon and acquires more energy than the work function
(the electron binding energy) of the material, it is ejected. If the
photon energy is too low, the electron is unable to escape the material.
Since an increase in the intensity of low-frequency light will only
increase the number of low-energy photons sent over a given interval of
time, this change in intensity will not create any single photon with
enough energy to dislodge an electron. Thus, the energy of the emitted
electrons does not depend on the intensity of the incoming light, but
only on the energy (equivalent frequency) of the individual photons. It
is an interaction between the incident photon and the outermost
electrons.
Electrons can absorb energy from photons when irradiated, but
they usually follow an "all or nothing" principle. All of the energy
from one photon must be absorbed and used to liberate one electron from
atomic binding, or else the energy is re-emitted. If the photon energy
is absorbed, some of the energy liberates the electron from the atom,
and the rest contributes to the electron's kinetic energy as a free particle.
Experimental observations of photoelectric emission
The
theory of the photoelectric effect must explain the experimental
observations of the emission of electrons from an illuminated metal
surface.
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, keeping the number of
incident photons fixed (this would result in a proportionate increase in
energy) increases the maximum kinetic energy of the photoelectrons emitted. Thus the stopping voltage increases (see the experimental setup in the figure). The number of electrons also changes because of the probability that each photon results in an emitted electron are a function of photon energy.
If the intensity of the incident radiation of a given frequency is
increased, there is no effect on the kinetic energy of each
photoelectron.
Above the threshold frequency, the maximum kinetic energy of the
emitted photoelectron depends on the frequency of the incident light,
but is independent of the intensity of the incident light so long as the
latter is not too high.
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. An increase in the intensity of the
incident beam (keeping the frequency fixed) increases the magnitude of
the photoelectric current, although the stopping voltage remains the
same.
The time lag between the incidence of radiation and the emission of a photoelectron is very small, less than 10−9 second.
The direction of distribution of emitted electrons peaks in the direction of polarization (the direction of the electric field) of the incident light, if it is linearly polarized.
Mathematical description
In 1905, Einstein proposed an explanation of the photoelectric effect
using a concept first put forward by Max Planck that light waves consist
of tiny bundles or packets of energy known as photons or quanta.
Diagram of the maximum kinetic energy as a function of the frequency of light on zinc
The maximum kinetic energy of an ejected electron is given by
where is the Planck constant and is the frequency of the incident photon. The term is the work function (sometimes denoted , or ), which gives the minimum energy required to remove an electron from the surface of the metal. The work function satisfies
where is the threshold frequency for the metal. The maximum kinetic energy of an ejected electron is then
Kinetic energy is positive, so we must have for the photoelectric effect to occur.
Stopping potential
The relation between current and applied voltage
illustrates the nature of the photoelectric effect. For discussion, a
light source illuminates a plate P, and another plate electrode Q
collects any emitted electrons. We vary the potential between P and Q
and measure the current flowing in the external circuit between the two
plates.
If the frequency and the intensity of the incident radiation are
fixed, the photoelectric current increases gradually with an increase in
the positive potential on the collector electrode until all the
photoelectrons emitted are collected. The photoelectric current attains a
saturation value and does not increase further for any increase in the
positive potential. The saturation current increases with the increase
of the light intensity. It also increases with greater frequencies due
to a greater probability of electron emission when collisions happen
with higher energy photons.
If we apply a negative potential
to the collector plate Q with respect to the plate P and gradually
increase it, the photoelectric current decreases, becoming zero at a
certain negative potential. The negative potential on the collector at
which the photoelectric current becomes zero is called the stopping potential or cut off potential.
- For a given frequency of incident radiation, the stopping potential is independent of its intensity.
- For a given frequency of incident radiation, the stopping potential is determined by the maximum kinetic energy of the photoelectrons that are emitted. If qe is the charge on the electron and is the stopping potential, then the work done by the retarding potential in stopping the electron is , so we have
Recalling
we see that the stopping voltage varies linearly with frequency of
light, but depends on the type of material. For any particular material,
there is a threshold frequency that must be exceeded, independent of
light intensity, to observe any electron emission.
Three-step model
In the X-ray regime, the photoelectric effect in crystalline material is often decomposed into three steps:
- Inner photoelectric effect (see photo diode below). The hole left behind can give rise to Auger effect, which is visible even when the electron does not leave the material. In molecular solids phonons are excited in this step and may be visible as lines in the final electron energy. The inner photoeffect has to be dipole allowed. The transition rules for atoms translate via the tight-binding model onto the crystal. They are similar in geometry to plasma oscillations in that they have to be transverse.
- Ballistic transport of half of the electrons to the surface. Some electrons are scattered.
- Electrons escape from the material at the surface.
In the three-step model, an electron can take multiple paths through
these three steps. All paths can interfere in the sense of the path integral formulation.
For surface states and molecules the three-step model does still make some sense as even most atoms have multiple electrons which can scatter the one electron leaving.
History
When a surface is exposed to electromagnetic radiation above a certain threshold frequency (typically visible light for alkali metals, near ultraviolet for other metals, and extreme ultraviolet for non-metals), the radiation is absorbed and electrons
are emitted.
Light, and especially ultra-violet light, discharges negatively
electrified bodies with the production of rays of the same nature as cathode rays. Under certain circumstances it can directly ionize gases. The first of these phenomena was discovered by Hertz and Hallwachs in 1887. The second was announced first by Philipp Lenard in 1900.
The ultra-violet light to produce these effects may be obtained from an arc lamp,
or by burning magnesium, or by sparking with an induction coil between
zinc or cadmium terminals, the light from which is very rich in
ultra-violet rays. Sunlight is not rich in ultra-violet rays, as these
have been absorbed by the atmosphere, and it does not produce nearly so
large an effect as the arc-light. Many substances besides metals
discharge negative electricity under the action of ultraviolet light:
lists of these substances will be found in papers by G. C. Schmidt and O. Knoblauch.
19th century
In 1839, Alexandre Edmond Becquerel discovered the photovoltaic effect while studying the effect of light on electrolytic cells. 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.
Johann Elster (1854–1920) and Hans Geitel (1855–1923), students in Heidelberg, developed the first practical photoelectric cells that could be used to measure the intensity of light. Elster and Geitel had investigated with great success the effects produced by light on electrified bodies.
In 1887, Heinrich Hertz observed the photoelectric effect and the production and reception of electromagnetic waves. He published these observations in the journal Annalen der Physik. His receiver 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 in
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. Hertz
concluded his months of investigation and reported the results
obtained. He did not further pursue the investigation of this effect.
The discovery by Hertz
in 1887 that the incidence of ultra-violet light on a spark gap
facilitated the passage of the spark, led immediately to a series of
investigations by Hallwachs, Hoor, Righi, and Stoletow
on the effect of light, and especially of ultra-violet light, on
charged bodies. It was proved by these investigations that a newly
cleaned surface of zinc, if charged with negative electricity, rapidly
loses this charge however small it may be when ultra-violet light falls
upon the surface; while if the surface is uncharged to begin with, it
acquires a positive charge when exposed to the light, the negative
electrification going out into the gas by which the metal is surrounded;
this positive electrification can be much increased by directing a
strong airblast against the surface. If however the zinc surface is
positively electrified it suffers no loss of charge when exposed to the
light: this result has been questioned, but a very careful examination
of the phenomenon by Elster and Geitel
has shown that the loss observed under certain circumstances is due to
the discharge by the light reflected from the zinc surface of negative
electrification on neighbouring conductors induced by the positive
charge, the negative electricity under the influence of the electric
field moving up to the positively electrified surface.
With regard to the Hertz effect, the researchers from the
start showed a great complexity of the phenomenon of photoelectric
fatigue — that is, the progressive diminution of the effect observed
upon fresh metallic surfaces. According to an important research by Wilhelm Hallwachs, ozone played an important part in the phenomenon.
However, other elements enter such as oxidation, the humidity, the mode
of polish of the surface, etc. It was at the time not even sure that
the fatigue is absent in a vacuum.
In the period from February 1888 and until 1891, a detailed analysis of photo effect was performed by Aleksandr Stoletov with results published in 6 works; four of them in Comptes Rendus, one review in Physikalische Revue (translated from Russian), and the last work in Journal de Physique.
First, in these works Stoletov invented a new experimental setup which
was more suitable for a quantitative analysis of photo effect. Using
this setup, he discovered the direct proportionality between the
intensity of light and the induced photo electric current (the first law
of photo effect or Stoletov's law).
One of his other findings resulted from measurements of the dependence
of the intensity of the electric photo current on the gas pressure,
where he found the existence of an optimal gas pressure Pm corresponding to a maximum photocurrent; this property was used for a creation of solar cells.
In 1899, J. J. Thomson investigated ultraviolet light in Crookes tubes.
Thomson deduced that the ejected particles were the same as those
previously found in the cathode ray, later called electrons, which he
called "corpuscles". In the research, Thomson enclosed a metal plate (a
cathode) in a vacuum tube, and exposed it to high-frequency radiation.
It was thought that the oscillating electromagnetic fields caused the
atoms' field to resonate and, after reaching a certain amplitude, caused
a subatomic "corpuscle" 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.
20th century
The discovery of the ionization of gases by ultra-violet 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 did J. J. Thomson, as a Hertz effect upon the solid or liquid particles present in the gas.
In 1902, Lenard observed that the energy of individual emitted electrons increased with the frequency (which is related to the color) of the light.
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 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, and that had sufficient power to
enable him to investigate the variation of potential with light
frequency. His experiment directly measured potentials, not electron
kinetic energy: he found the electron energy by relating it to the
maximum stopping potential (voltage) in a phototube. He found that the
calculated maximum electron kinetic energy
is determined by the frequency of the light. For example, an increase
in frequency results in an increase in the maximum kinetic energy
calculated for an electron upon liberation – ultraviolet radiation
would require a higher applied stopping potential to stop current in a
phototube than blue light. 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.
The researches of Langevin and those of Eugene Bloch have shown that the greater part of the Lenard effect is certainly due to this 'Hertz effect'. The Lenard effect upon the gas itself nevertheless does exist. Refound by J. J. Thomson and then more decisively by Frederic Palmer, Jr., it was studied and showed very different characteristics than those at first attributed to it by Lenard.
In 1905, Albert Einstein solved this apparent paradox by describing light as composed of discrete quanta, now called photons, rather than continuous waves. Based upon Max Planck's theory of black-body radiation, Einstein theorized that the energy in each quantum of light was equal to the frequency multiplied by a constant, later called Planck's constant.
A photon above a threshold frequency has the required energy to eject a
single electron, creating the observed effect. This discovery led to
the quantum revolution in physics and earned Einstein the Nobel Prize in Physics in 1921. By wave-particle duality the effect can be analyzed purely in terms of waves though not as conveniently.
Albert Einstein's mathematical description of how the photoelectric effect was caused by absorption of quanta of light was in one of his 1905 papers, named "On a Heuristic Viewpoint Concerning the Production and Transformation of Light".
This paper proposed the simple description of "light quanta", or
photons, and showed how they explained such phenomena as the
photoelectric effect. His simple explanation in terms of absorption of
discrete quanta of light explained the features of the phenomenon and the characteristic frequency.
The idea of light quanta began with Max Planck's published law of black-body radiation ("On the Law of Distribution of Energy in the Normal Spectrum") by assuming that Hertzian oscillators could only exist at energies E proportional to the frequency f of the oscillator by E = hf, where h
is Planck's constant. By assuming that light actually consisted of
discrete energy packets, Einstein wrote an equation for the
photoelectric effect that agreed with experimental results. It explained
why the energy of photoelectrons was dependent only on the frequency of the incident light and not on its intensity:
a low-intensity, the high-frequency source could supply a few high
energy photons, whereas a high-intensity, the low-frequency source would
supply no photons of sufficient individual energy to dislodge any
electrons. This was an enormous 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 for electromagnetic behavior, and more generally, the assumption of infinite divisibility
of energy in physical systems. Even after experiments showed that
Einstein's equations for the photoelectric effect were accurate, resistance to the idea of photons continued since it appeared to contradict Maxwell's equations, which were well understood and verified.
Einstein's work predicted that the energy of individual ejected
electrons increases linearly with the frequency of the light. Perhaps
surprisingly, 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 Robert Andrews Millikan showed that Einstein's prediction was correct.
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,
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.
Uses and effects
Photomultipliers
These are extremely light-sensitive vacuum tubes with a photocathode
coated onto part (an end or side) of the inside of the envelope. The
photo cathode contains combinations of materials such as cesium,
rubidium, and antimony specially selected to provide a low work
function, so when illuminated even by very low levels of light, the
photocathode readily releases electrons. By means of a series of
electrodes (dynodes) at ever-higher potentials, these electrons are
accelerated and substantially increased in number through secondary emission
to provide a readily detectable output current. Photomultipliers are
still commonly used wherever low levels of light must be detected.
Image sensors
Video camera tubes in the early days of television used the photoelectric effect, for example, Philo Farnsworth's "Image dissector" used a screen charged by the photoelectric effect to transform an optical image into a scanned electronic signal.
Gold-leaf electroscope
The gold leaf electroscope
Gold-leaf electroscopes are designed to detect static electricity. Charge placed on the metal cap spreads to the stem and the gold leaf of the electroscope. Because they then have the same charge, the stem and leaf repel each other. This will cause the leaf to bend away from the stem.
An electroscope is an important tool in illustrating the
photoelectric effect. For example, if the electroscope is negatively
charged throughout, there is an excess of electrons and the leaf is
separated from the stem. If high-frequency light shines on the cap, the
electroscope discharges, and the leaf will fall limp. This is because
the frequency of the light shining on the cap is above the cap's
threshold frequency. The photons in the light have enough energy to
liberate electrons from the cap, reducing its negative charge. This will
discharge a negatively charged electroscope and further charge a
positive electroscope. However, if the electromagnetic radiation hitting
the metal cap does not have a high enough frequency (its frequency is
below the threshold value for the cap), then the leaf will never
discharge, no matter how long one shines the low-frequency light at the
cap.
Photoelectron spectroscopy
Since
the energy of the photoelectrons emitted is exactly the energy of the
incident photon minus the material's work function or binding energy,
the work function of a sample can be determined by bombarding it with a monochromatic X-ray source or UV source, and measuring the kinetic energy distribution of the electrons emitted.
Photoelectron spectroscopy
is usually done in a high-vacuum environment, since the electrons would
be scattered by gas molecules if they were present. However, some
companies are now selling products that allow photoemission in air. The
light source can be a laser, a discharge tube, or a synchrotron radiation source.
The concentric hemispherical analyzer is a typical electron
energy analyzer and uses an electric field to change the directions of
incident electrons, depending on their kinetic energies. For every
element and core (atomic orbital) there will be a different binding
energy. The many electrons created from each of these combinations will
show up as spikes in the analyzer output, and these can be used to
determine the elemental composition of the sample.
Spacecraft
The photoelectric effect will cause spacecraft
exposed to sunlight to develop a positive charge. This can be a major
problem, as other parts of the spacecraft are in shadow which will
result in the spacecraft developing a negative charge from nearby
plasmas. The imbalance can discharge through delicate electrical
components. The static charge
created by the photoelectric effect is self-limiting, because a higher
charged object doesn't give up its electrons as easily as a lower
charged object does.
Moon dust
Light
from the sun hitting lunar dust causes it to become charged with the
photoelectric effect. The charged dust then repels itself and lifts off
the surface of the Moon by electrostatic levitation.
This manifests itself almost like an "atmosphere of dust", visible as a
thin haze and blurring of distant features, and visible as a dim glow
after the sun has set. This was first photographed by the Surveyor program
probes in the 1960s. It is thought that the smallest particles are
repelled kilometers from the surface and that the particles move in
"fountains" as they charge and discharge.
Night vision devices
Photons hitting a thin film of alkali metal or semiconductor material such as gallium arsenide in an image intensifier tube cause the ejection of photoelectrons due to the photoelectric effect. These are accelerated by an electrostatic field where they strike a phosphor
coated screen, converting the electrons back into photons.
Intensification of the signal is achieved either through acceleration of
the electrons or by increasing the number of electrons through
secondary emissions, such as with a micro-channel plate.
Sometimes a combination of both methods is used. Additional kinetic
energy is required to move an electron out of the conduction band and
into the vacuum level. This is known as the electron affinity of the photocathode and is another barrier to photoemission other than the forbidden band, explained by the band gap
model. Some materials such as Gallium Arsenide have an effective
electron affinity that is below the level of the conduction band. In
these materials, electrons that move to the conduction band are all of
the sufficient energy to be emitted from the material and as such, the
film that absorbs photons can be quite thick. These materials are known
as negative electron affinity materials.
Cross section
The
photoelectric effect is one interaction mechanism between photons and
atoms. It is one of 12 theoretically possible interactions.
At the high photon energies comparable to the electron rest energy of 511 keV, Compton scattering, another process, may take place. Above twice this (1.022 MeV) pair production may take place. Compton scattering and pair production are examples of two other competing mechanisms.
Indeed, even if the photoelectric effect is the favoured reaction
for a particular single-photon bound-electron interaction, the result
is also subject to statistical processes and is not guaranteed, albeit
the photon has certainly disappeared and a bound electron has been
excited (usually K or L shell electrons at gamma ray
energies). The probability of the photoelectric effect occurring is
measured by the cross-section of interaction, σ. This has been found to
be a function of the atomic number of the target atom and photon energy.
A crude approximation, for photon energies above the highest atomic
binding energy, is given by:
Here Z is atomic number and n
is a number which varies between 4 and 5. (At lower photon energies a
characteristic structure with edges appears, K edge, L edges, M edges,
etc.) The obvious interpretation follows that the photoelectric effect
rapidly decreases in significance, in the gamma-ray region of the
spectrum, with increasing photon energy, and that photoelectric effect
increases steeply with atomic number. The corollary is that high-Z materials make good gamma-ray shields, which is the principal reason that lead (Z = 82) is a preferred and ubiquitous gamma radiation shield.
