spectra of opaque materials, such as
powders, are obtained by exposing
the material to light that is modu-
lated 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 chem-
ical reaction caused by light or ultra-
violet radiation. The incident
photons are absorbed by reactant
molecules to give excited molecules
or free radicals, which undergo fur-
ther reaction.
photochemical smogA noxious
smog produced by the reaction of ni-
trogen oxides with hydrocarbons in
the presence of ultraviolet light from
the sun. The reaction is very complex
and one of the products is ozone.
photochemistryThe branch of
chemistry concerned with *photo-
chemical reactions.
photochromism A change of col-
our occurring in certain substances
when exposed to light. Photochromic
materials are used in sunglasses that
darken in bright sunlight.
photoconductive effect See pho-
toelectric effect.
photoelectric effect The libera-
tion of electrons from a substance ex-
posed to electromagnetic radiation.
The number of such electrons (pho-
toelectrons) emitted depends on the
intensity of the radiation. The kinetic
energy of the electrons emitted de-
pends on the frequency of the radia-
tion. 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 con-
stant and f is the frequency of the ra-
diation. A photon can only eject an
electron if the photon energy ex-
ceeds the *work function, φ, of the
solid, i.e. if hf 0 = φan 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 ul-
traviolet frequencies or above, but
for some materials (having low work
functions) it occurs with light. The
maximum kinetic energy, Em, of the
photoelectron is given by *Einstein’s
equation: Em= hf– φ.
Apart from the liberation of elec-
trons from atoms, other phenomena
are also referred to as photoelectric
effects. These are the photoconduc-
tive effect and the photovoltaic ef-
fect. In the photoconductive effect,
an increase in the electrical conduc-
tivity of a semiconductor is caused by
radiation as a result of the excitation
of additional free charge carriers by
the incident photons. Photoconduc-
tive 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 irra-
diation. The effect is made use of in
photovoltaic cells, most of which
consist of p–n semiconductor junc-
tions. When photons are absorbed
near a p–n junction new free charge
carriers are produced (as in photo-
conductivity); however, in the photo-
voltaic effect the electricÜeld 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 emit-
ted from a substance by irradiation
as a result of the *photoelectric ef-
fect or *photoionization.
photoelectron spectromicros-
copy(PESM)A technique for inves-415 photoelectron spectromicroscopy
p