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 disappear-
ing in absorption. Absorption occurs
when electromagnetic radiation that
impinges on a quantum mechanical
system, such as an atom or mol-
ecules, induces a transition from the
ground state of the system to an ex-
cited state by virtue of a photon of
the radiation being absorbed. Simi-
larly, 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 ex-
cited state to emit a photon by in-
duced emission, a process that is
used in *lasers.
quantum yield In a photochemi-
cal reaction, the number of events
occurring per photon absorbed. It is
given by the number of moles of
product formed (or reactant con-
sumed) divided by the number of
moles of photons absorbed.
quartz The most abundant and
common mineral, consisting of crys-
talline 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 ordinar-
ily colourless and transparent, in
which form it is known as rock crys-
tal. Coloured varieties, a number of
which are used as gemstones, in-
clude amethyst, citrine quartz (yel-
low), rose quartz (pink), milk quartz
(white), smoky quartz (grey-brown),
chalcedony, agate, and jasper.
Quartz occurs in many rocks, espe-
cially igneous rocks such as granite
and quartzite (of which it is the chief
constituent), metamorphic rocks
such as gneisses and schists, and sedi-
mentary rocks such as sandstone and
limestone. The mineral is piezoelec-
tric and is used in oscillators. It is
also used in optical instruments and
in glass, glaze, and abrasives.
quasicrystalA solid structure in
which there is (1) long-range incom-
mensurate translational order (see in-
commensurate lattice) and (2)
long-range orientational order with a
point group, which is not allowed in
*crystallography. Condition (1) is
called quasiperiodicity. In two dimen-
sions, theÜvefold symmetry of a pen-
tagon is an example of a point
symmetry, which is not allowed in
crystallography, but for which qua-
sicrystals 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.quasiperiodicitySee quasicrystal.
quaternary ammonium com-
poundsSee amine salts.quenching1.(in metallurgy) The
rapid cooling of a metal by immers-
ing 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.quicklimeSee calcium oxide.449 quicklime
q