spectra of opaque materials, such as
powders, are obtained by exposing
the material to light that is modulated at acoustic frequencies. This
produces a photoacoustic signal, the
magnitude of which can be related to
the ultraviolet or infrared absorption
coefÜcient of the material. See also
optoacoustic spectroscopy.
photochemical reaction A chemical reaction caused by light or ultraviolet radiation. The incident
photons are absorbed by reactant
molecules to give excited molecules
or free radicals, which undergo further reaction.
photochemical smog A noxious
smog produced by the reaction of nitrogen oxides with hydrocarbons in
the presence of ultraviolet light from
the sun. The reaction is very complex
and one of the products is ozone.
photochemistry The branch of
chemistry concerned with *photochemical reactions.
photochromism A change of colour occurring in certain substances
when exposed to light. Photochromic
materials are used in sunglasses that
darken in bright sunlight.
photoconductive effect See photoelectric effect.
photoelectric effect The liberation of electrons from a substance exposed to electromagnetic radiation.
The number of such electrons (photoelectrons) emitted depends on the
intensity of the radiation. The kinetic
energy of the electrons emitted depends on the frequency of the radiation. The effect is a quantum process
in which the radiation is regarded as
a stream of *photons, each having an
energy hf, where h is the Planck constant and f is the frequency of the radiation. A photon can only eject an
electron if the photon energy exceeds the *work function, φ, of the
solid, i.e. if hf 0 = φ an electron will be
ejected; f 0 is the minimum frequency
(or threshold frequency) at which
ejection will occur. For many solids
the photoelectric effect occurs at ultraviolet frequencies or above, but
for some materials (having low work
functions) it occurs with light. The
maximum kinetic energy, E m , of the
photoelectron is given by *Einstein’s
equation: E m = hf – φ.
Apart from the liberation of electrons from atoms, other phenomena
are also referred to as photoelectric
effects. These are the photoconductive effect and the photovoltaic effect. In the photoconductive effect,
an increase in the electrical conductivity of a semiconductor is caused by
radiation as a result of the excitation
of additional free charge carriers by
the incident photons. Photoconductive cells, using such photosensitive
materials as cadmium sulphide, are
widely used as radiation detectors
and light switches (e.g. to switch on
street lighting).
In the photovoltaic effect, an e.m.f.
is produced between two layers of
different materials as a result of irradiation. The effect is made use of in
photovoltaic cells, most of which
consist of p–n semiconductor junctions. When photons are absorbed
near a p–n junction new free charge
carriers are produced (as in photoconductivity); however, in the photovoltaic effect the electric Üeld in the
junction region causes the new
charge carriers to move, creating a
Ûow of current in an external circuit
without the need for a battery.
photoelectron An electron emitted from a substance by irradiation
as a result of the *photoelectric effect or *photoionization.
photoelectron spectromicroscopy (PESM) A technique for inves415
photoelectron spectromicroscopy
p
www.AzShimi.ir www.AzShimi.com
powders, are obtained by exposing
the material to light that is modulated at acoustic frequencies. This
produces a photoacoustic signal, the
magnitude of which can be related to
the ultraviolet or infrared absorption
coefÜcient of the material. See also
optoacoustic spectroscopy.
photochemical reaction A chemical reaction caused by light or ultraviolet radiation. The incident
photons are absorbed by reactant
molecules to give excited molecules
or free radicals, which undergo further reaction.
photochemical smog A noxious
smog produced by the reaction of nitrogen oxides with hydrocarbons in
the presence of ultraviolet light from
the sun. The reaction is very complex
and one of the products is ozone.
photochemistry The branch of
chemistry concerned with *photochemical reactions.
photochromism A change of colour occurring in certain substances
when exposed to light. Photochromic
materials are used in sunglasses that
darken in bright sunlight.
photoconductive effect See photoelectric effect.
photoelectric effect The liberation of electrons from a substance exposed to electromagnetic radiation.
The number of such electrons (photoelectrons) emitted depends on the
intensity of the radiation. The kinetic
energy of the electrons emitted depends on the frequency of the radiation. The effect is a quantum process
in which the radiation is regarded as
a stream of *photons, each having an
energy hf, where h is the Planck constant and f is the frequency of the radiation. A photon can only eject an
electron if the photon energy exceeds the *work function, φ, of the
solid, i.e. if hf 0 = φ an electron will be
ejected; f 0 is the minimum frequency
(or threshold frequency) at which
ejection will occur. For many solids
the photoelectric effect occurs at ultraviolet frequencies or above, but
for some materials (having low work
functions) it occurs with light. The
maximum kinetic energy, E m , of the
photoelectron is given by *Einstein’s
equation: E m = hf – φ.
Apart from the liberation of electrons from atoms, other phenomena
are also referred to as photoelectric
effects. These are the photoconductive effect and the photovoltaic effect. In the photoconductive effect,
an increase in the electrical conductivity of a semiconductor is caused by
radiation as a result of the excitation
of additional free charge carriers by
the incident photons. Photoconductive cells, using such photosensitive
materials as cadmium sulphide, are
widely used as radiation detectors
and light switches (e.g. to switch on
street lighting).
In the photovoltaic effect, an e.m.f.
is produced between two layers of
different materials as a result of irradiation. The effect is made use of in
photovoltaic cells, most of which
consist of p–n semiconductor junctions. When photons are absorbed
near a p–n junction new free charge
carriers are produced (as in photoconductivity); however, in the photovoltaic effect the electric Üeld in the
junction region causes the new
charge carriers to move, creating a
Ûow of current in an external circuit
without the need for a battery.
photoelectron An electron emitted from a substance by irradiation
as a result of the *photoelectric effect or *photoionization.
photoelectron spectromicroscopy (PESM) A technique for inves415
photoelectron spectromicroscopy
p
www.AzShimi.ir www.AzShimi.com