scattering theory for short-range
order.
excimerSee exciplex.exciplexA combination of two dif-
ferent atoms that exists only in an
excited state. When an exciplex
emits a photon of electromagnetic ra-
diation, it immediately dissociates
into the atoms, rather than reverting
to the ground state. A similar tran-
sient excited association of two
atoms of the same kind is an
excimer. An example of an exciplex
is the species XeCl*(the asterisk indi-
cates an excited state), which can be
formed by an electric discharge in
xenon and chlorine. This is used in
the exciplex laser, in which a popula-
tion inversion is produced by an elec-
trical discharge.
excitationA process in which a nu-
cleus, electron, atom, ion, or mol-
ecule acquires energy that raises it to
a quantum state (excited state)
higher than that of its *ground state.
The difference between the energy in
the ground state and that in the ex-
cited state is called the excitation en-
ergy. See energy level.exclusion principleSee pauli ex-
clusion principle.exo-See endo-.
exothermicDenoting a chemical
reaction that releases heat into its
surroundings. Compare endothermic.exotic atomA species in which
some other charged particle replaces
the electron or nucleon. An example
is an atom in which an electron has
been replaced by another negatively
charged particle, such as a muon or
meson. In this case the negative par-
ticle eventually collides with the nu-
cleus with the emission of X-ray
photons. Another system is one in
which the nucleus of an atom has
been replaced by a positively chargedmeson. An association of an electron
and a positron is also regarded as an
exotic atom (known as positronium).
This has a mean life of about 10–75,
decaying to give three photons.
expanded plastic (cellular plastic)
A plastic in the form of a rigid solid
foam. Polystyrene, used as a packing
material, is a common example.explosiveA compound or mixture
that, when ignited or detonated, un-
dergoes a rapid violent chemical re-
action that produces large amounts
of gas and heat, accompanied by
light, sound and a high-pressure
shock wave. Low explosives burn
comparatively slowly when ignited,
and are employed as propellants in
Ürearms and guns; they are also used
in blasting. Examples include *gun-
powder and various smokeless pro-
pellants, such as *cordite. High
explosivesdecompose very rapidly
to produce an uncontrollable blast.
Examples of this type include *dyna-
mite, *nitroglycerine, and *trinitro-
toluene (TNT); they are exploded
using a detonator. Other high-power
explosives include pentaerythritol
tetranitrate (PETN) and ammonium
nitride/fuel oil mixture (ANFO). Cy-
clonite (RDX) is a military high explo-
sive; mixed with oils and waxes, it
forms a plastic explosive (such as
Semtex). See also Chronology.extended X-ray absorption Üne
structure See exafs.extenderAn inert substance added
to a product (paint, rubber, washing
powder, etc.) to dilute it (for econ-
omy) or to modify its physical prop-
erties.
extensive variableA quantity in
a *macroscopic system that is pro-
portional to the size of the system.
Examples of extensive variables in-
clude the volume, mass, and total en-
ergy. If an extensive variable isexcimer 216e