and absorption occurs only in the
surface layer. ATRS is used in foren-
sic science for analysis of thin layers
(e.g. paint).
atto- Symbol a. A preÜx used in the
metric system to denote 10–18. For ex-
ample, 10–18second = 1 attosecond
(as).
attractor The set of points in phase
space to which the representative
point of a dissipative system (i.e. one
with internal friction) tends as the
system evolves. The attractor can be:
a single point; a closed curve (a limit
cycle), which describes a system with
periodic behaviour; or a strange at-
tractor, in which case the system ex-
hibits *chaos.
Aufbau principleA principle that
gives the order in which orbitals are
Ülled in successive elements in the
periodic table. The order ofÜlling is
1 s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p,
6 s, 4f, 5d, 6p, 7s, 5f, 6d. See atom.
Auger effect The ejection of an
electron from an atom as a result of
the de-excitation of an excited elec-
tron within the atom. An electron is
Ürst ejected from an atom by a pho-
ton, electron impact, ion impact, or
some other process, thus creating a
vacancy. In the subsequent re-
arrangement of the electronic struc-
ture of the atom, an electron from a
higher energy level falls into the va-
cancy. This process is associated with
excess energy, which is released by
the ejection of a second electron
(rather than by emission of a pho-
ton). This second electron is called
the Auger electron. Auger spectros-
copyis a form of electron spectros-
copy using this effect to study the
energy levels of ions. It is also a form
of analysis and can be used to iden-
tify the presence of elements in sur-
face layers of solids. The effect was
discovered by the French physicist
Pierre Auger (1899–1994) in 1925.Auger electron See auger effect.auric compoundsCompounds of
gold in its higher (+3) oxidation state;
e.g. auric chloride is gold(III) chloride
(AuCl 3 ).aurous compounds Compounds
of gold in its lower (+1) oxidation
state; e.g. aurous chloride is gold(I)
chloride (AuCl).austeniteSee steel.autocatalysis*Catalysis in which
one of the products of the reaction is
a catalyst for the reaction. Reactions
in which autocatalysis occurs have a
characteristic S-shaped curve for re-
action rate against time – the reac-
tion starts slowly and increases as
the amount of catalyst builds up,
falling off again as the products are
used up.autoclaveA strong steel vessel
used for carrying out chemical reac-
tions, sterilizations, etc., at high tem-
perature and pressure.automated tensor low-energy
electron diffraction See atleed.autoprotolysis A transfer of a hy-
drogen ion (H+) between molecules of
an amphiprotic *solvent, one mol-
ecule acting as a Brønsted acid and
the other as a Brønsted base. It oc-
curs in the autoionization of water.autoprotolysis constant See
ionic product.autoradiographyAn experimen-
tal technique in which a radioactive
specimen is placed in contact with
(or close to) a photographic plate, so
as to produce a record of the distribu-
tion of radioactivity in the specimen.
TheÜlm is darkened by the ionizing
radiation from radioactive parts of
the sample. Autoradiography has a51 autoradiography
a