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=                        Photoelectric effect                        =
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                             Introduction
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The photoelectric effect is the emission of electrons or other free
carriers when light hits a material. Electrons emitted in this manner
can be called 'photoelectrons'. This phenomenon is commonly studied in
electronic physics and in fields of chemistry such as quantum
chemistry and electrochemistry.

According to classical electromagnetic theory, the photoelectric
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 induces changes in the kinetic energy of the electrons emitted
from the metal. According to this theory, a sufficiently dim light is
expected to show a time lag between the initial shining of its light
and the subsequent emission of an electron.

But the experimental results did not correlate with either of the two
predictions made by classical theory. Instead, experiments showed that
electrons are dislodged only by the impingement of light when it
reached or exceeded a threshold frequency. 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.

Because a low-frequency beam at a high intensity could not build up
the energy required to produce photoelectrons like it would have if
light's energy were continuous like a wave, Einstein proposed that a
beam of light is not a wave propagating through space, but rather a
collection of discrete wave packets (photons).

Emission of conduction electrons from typical metals usually requires
a few electron-volts, corresponding to short-wavelength visible or
ultraviolet light. Emissions can be induced with 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.  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.


                          Emission mechanism
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The photons of a light beam have a characteristic energy which is
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 innermost
electrons. The movement of an outer electron to occupy the vacancy
then result in the emission of a photon.

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.

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'.


 Experimental observations of photoelectric emission
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The theory of the source of 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