secondary-ion mass spectrom-
etry(SIMS)A technique for ana-
lysing the chemical structure of a
solid by bombarding it with ions. A
sample of the solid to be analysed is
bombarded with ions, referred to as
primary ions, having energies be-
tween 5 and 20 keV. A number of dif-
ferent types of entity are detached
from the surface of the solid, includ-
ing neutral atoms and molecules,
electrons, photons, and negative and
positive ions. The negative and posi-
tive ions given off, called the sec-
ondary ions, can be identiÜed using
mass spectrometry. Using this tech-
nique solid samples can be character-
ized with great accuracy.second-order reaction See order.
sedimentationThe settling of the
solid particles through a liquid either
to produce a concentrated slurry
from a dilute suspension or to clarify
a liquid containing solid particles.
Usually this relies on the force of
gravity, but if the particles are too
small or the difference in density be-
tween the solid and liquid phases is
too small, a *centrifuge may be used.
In the simplest case the rate of sedi-
mentation is determined by Stokes’s
law, but in practice the predicted
rate is rarely reached. Measurement
of the rate of sedimentation in an
*ultracentrifuge can be used to esti-
mate the size of macromolecules.seedA crystal used to induce other
crystals to form from a gas, liquid, or
solution.Seger cones (pyrometric cones) A
series of cones used to indicate the
temperature inside a furnace or kiln.
The cones are made from different
mixtures of clay, limestone, feld-
spars, etc., and each one softens at a
different temperature. The drooping
of the vertex is an indication that the
known softening temperature hasbeen reached and thus the furnace
temperature can be estimated.selection rulesRules that deter-
mine which transitions between dif-
ferent energy levels are possible
in a system, such as an elementary
particle, nucleus, atom, molecule, or
crystal, described by quantum me-
chanics. Transitions cannot take
place between any two energy levels.
Group theory, associated with the
symmetry of the system, determines
which transitions, called *allowed
transitions, can take place and
which transitions, called *forbidden
transitions, cannot take place. Se-
lection rules determined in this
way are very useful in analysing the
spectra of quantum-mechanical sys-
tems.
selenidesBinary compounds of
selenium with other more elec-
tropositive elements. Selenides of
nonmetals are covalent (e.g. H 2 Se).
Most metal selenides can be pre-
pared by direct combination of the
elements. Some are well-deÜned
ionic compounds (containing Se2–),
while others are nonstoichiometric
interstitial compounds (e.g. Pd 4 Se,
PdSe 2 ).
seleniumSymbol Se. A metalloid
element belonging to group 16 (for-
merly VIB) of the periodic table; a.n.
34; r.a.m. 78.96; r.d. 4.81 (grey); m.p.
217 °C (grey); b.p. 684.9°C. There are a
number of allotropic forms, includ-
ing grey, red, and black selenium. It
occurs in sulphide ores of other met-
als and is obtained as a by-product
(e.g. from the anode sludge in elec-
trolytic reÜning). The element is a
semiconductor; the grey allotrope is
light-sensitive and is used in photo-
cells, xerography, and similar appli-
cations. Chemically, it resembles
sulphur, and forms compounds with
selenium in the +2, +4, and +6 oxida-secondary-ion mass spectrometry 476s