theory the processes of emission and
absorption are described in terms of
the creation and disappearance of
photons, with photons being created
in emission processes and disappearing in absorption. Absorption occurs
when electromagnetic radiation that
impinges on a quantum mechanical
system, such as an atom or molecules, induces a transition from the
ground state of the system to an excited state by virtue of a photon of
the radiation being absorbed. Similarly, emission of a photon occurs
when an atom returns to the ground
state from an excited state, i.e.
*spontaneous emission occurs. It is
also possible for an atom in an excited state to emit a photon by *induced emission, a process that is
used in *lasers.
quantum yield In a photochemical reaction, the number of events
occurring per photon absorbed. It is
given by the number of moles of
product formed (or reactant consumed) divided by the number of
moles of photons absorbed.
quartz The most abundant and
common mineral, consisting of crystalline silica (silicon dioxide, SiO 2 ),
crystallizing in the trigonal system. It
has a hardness of 7 on the Mohs’
scale. Well-formed crystals of quartz
are six-sided prisms terminating in
six-sided pyramids. Quartz is ordinarily colourless and transparent, in
which form it is known as rock crystal. Coloured varieties, a number of
which are used as gemstones, include *amethyst, citrine quartz (yellow), rose quartz (pink), milk quartz
(white), smoky quartz (grey-brown),
*chalcedony, *agate, and *jasper.
Quartz occurs in many rocks, especially igneous rocks such as granite
and quartzite (of which it is the chief
constituent), metamorphic rocks
such as gneisses and schists, and sedimentary rocks such as sandstone and
limestone. The mineral is piezoelectric and is used in oscillators. It is
also used in optical instruments and
in glass, glaze, and abrasives.
quasicrystal A solid structure in
which there is (1) long-range incommensurate translational order (see incommensurate lattice) and (2)
long-range orientational order with a
point group, which is not allowed in
*crystallography. Condition (1) is
called quasiperiodicity. In two dimensions, the Üvefold symmetry of a pentagon is an example of a point
symmetry, which is not allowed in
crystallography, but for which quasicrystals exist. In three dimensions
the symmetry of an icosahedron is
not allowed in crystallography, but
quasicrystals with this symmetry
exist (e.g. AlMn). Diffraction patterns
for quasicrystals have Bragg peaks,
with the density of Bragg peaks in
each plane being higher than would
be expected for a perfect periodic
crystal.
quasiperiodicity See quasicrystal.
quaternary ammonium compounds See amine salts.
quenching 1. (in metallurgy) The
rapid cooling of a metal by immersing it in a bath of liquid in order to
improve its properties. Steels are
quenched to make them harder
but some nonferrous metals are
quenched for other reasons (copper,
for example, is made softer by
quenching). 2. (in physics) The
process of inhibiting a continuous
discharge in a *Geiger counter so
that the incidence of further ionizing
radiation can cause a new discharge.
This is achieved by introducing a
quenching vapour, such as methane
mixed with argon or with neon, into
the tube.
quicklime See calcium oxide.
449
quicklime
q
www.AzShimi.ir www.AzShimi.com
absorption are described in terms of
the creation and disappearance of
photons, with photons being created
in emission processes and disappearing in absorption. Absorption occurs
when electromagnetic radiation that
impinges on a quantum mechanical
system, such as an atom or molecules, induces a transition from the
ground state of the system to an excited state by virtue of a photon of
the radiation being absorbed. Similarly, emission of a photon occurs
when an atom returns to the ground
state from an excited state, i.e.
*spontaneous emission occurs. It is
also possible for an atom in an excited state to emit a photon by *induced emission, a process that is
used in *lasers.
quantum yield In a photochemical reaction, the number of events
occurring per photon absorbed. It is
given by the number of moles of
product formed (or reactant consumed) divided by the number of
moles of photons absorbed.
quartz The most abundant and
common mineral, consisting of crystalline silica (silicon dioxide, SiO 2 ),
crystallizing in the trigonal system. It
has a hardness of 7 on the Mohs’
scale. Well-formed crystals of quartz
are six-sided prisms terminating in
six-sided pyramids. Quartz is ordinarily colourless and transparent, in
which form it is known as rock crystal. Coloured varieties, a number of
which are used as gemstones, include *amethyst, citrine quartz (yellow), rose quartz (pink), milk quartz
(white), smoky quartz (grey-brown),
*chalcedony, *agate, and *jasper.
Quartz occurs in many rocks, especially igneous rocks such as granite
and quartzite (of which it is the chief
constituent), metamorphic rocks
such as gneisses and schists, and sedimentary rocks such as sandstone and
limestone. The mineral is piezoelectric and is used in oscillators. It is
also used in optical instruments and
in glass, glaze, and abrasives.
quasicrystal A solid structure in
which there is (1) long-range incommensurate translational order (see incommensurate lattice) and (2)
long-range orientational order with a
point group, which is not allowed in
*crystallography. Condition (1) is
called quasiperiodicity. In two dimensions, the Üvefold symmetry of a pentagon is an example of a point
symmetry, which is not allowed in
crystallography, but for which quasicrystals exist. In three dimensions
the symmetry of an icosahedron is
not allowed in crystallography, but
quasicrystals with this symmetry
exist (e.g. AlMn). Diffraction patterns
for quasicrystals have Bragg peaks,
with the density of Bragg peaks in
each plane being higher than would
be expected for a perfect periodic
crystal.
quasiperiodicity See quasicrystal.
quaternary ammonium compounds See amine salts.
quenching 1. (in metallurgy) The
rapid cooling of a metal by immersing it in a bath of liquid in order to
improve its properties. Steels are
quenched to make them harder
but some nonferrous metals are
quenched for other reasons (copper,
for example, is made softer by
quenching). 2. (in physics) The
process of inhibiting a continuous
discharge in a *Geiger counter so
that the incidence of further ionizing
radiation can cause a new discharge.
This is achieved by introducing a
quenching vapour, such as methane
mixed with argon or with neon, into
the tube.
quicklime See calcium oxide.
449
quicklime
q
www.AzShimi.ir www.AzShimi.com