secondary-ion mass spectrometry (SIMS) A technique for analysing 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 between 5 and 20 keV. A number of different types of entity are detached
from the surface of the solid, including neutral atoms and molecules,
electrons, photons, and negative and
positive ions. The negative and positive ions given off, called the secondary ions, can be identiÜed using
mass spectrometry. Using this technique solid samples can be characterized with great accuracy.
second-order reaction See order.
sedimentation The 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 between the solid and liquid phases is
too small, a *centrifuge may be used.
In the simplest case the rate of sedimentation 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 estimate the size of macromolecules.
seed A 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, feldspars, etc., and each one softens at a
different temperature. The drooping
of the vertex is an indication that the
known softening temperature has
been reached and thus the furnace
temperature can be estimated.
selection rules Rules that determine which transitions between different energy levels are possible
in a system, such as an elementary
particle, nucleus, atom, molecule, or
crystal, described by quantum mechanics. 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. Selection rules determined in this
way are very useful in analysing the
spectra of quantum-mechanical systems.
selenides Binary compounds of
selenium with other more electropositive elements. Selenides of
nonmetals are covalent (e.g. H 2 Se).
Most metal selenides can be prepared by direct combination of the
elements. Some are well-deÜned
ionic compounds (containing Se
2– ),
while others are nonstoichiometric
interstitial compounds (e.g. Pd 4 Se,
PdSe 2 ).
selenium Symbol Se. A metalloid
element belonging to group 16 (formerly 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, including grey, red, and black selenium. It
occurs in sulphide ores of other metals and is obtained as a by-product
(e.g. from the anode sludge in electrolytic reÜning). The element is a
semiconductor; the grey allotrope is
light-sensitive and is used in photocells, xerography, and similar applications. Chemically, it resembles
sulphur, and forms compounds with
selenium in the +2, +4, and +6 oxidasecondary-ion mass spectrometry
476
s
www.AzShimi.ir www.AzShimi.com
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 between 5 and 20 keV. A number of different types of entity are detached
from the surface of the solid, including neutral atoms and molecules,
electrons, photons, and negative and
positive ions. The negative and positive ions given off, called the secondary ions, can be identiÜed using
mass spectrometry. Using this technique solid samples can be characterized with great accuracy.
second-order reaction See order.
sedimentation The 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 between the solid and liquid phases is
too small, a *centrifuge may be used.
In the simplest case the rate of sedimentation 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 estimate the size of macromolecules.
seed A 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, feldspars, etc., and each one softens at a
different temperature. The drooping
of the vertex is an indication that the
known softening temperature has
been reached and thus the furnace
temperature can be estimated.
selection rules Rules that determine which transitions between different energy levels are possible
in a system, such as an elementary
particle, nucleus, atom, molecule, or
crystal, described by quantum mechanics. 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. Selection rules determined in this
way are very useful in analysing the
spectra of quantum-mechanical systems.
selenides Binary compounds of
selenium with other more electropositive elements. Selenides of
nonmetals are covalent (e.g. H 2 Se).
Most metal selenides can be prepared by direct combination of the
elements. Some are well-deÜned
ionic compounds (containing Se
2– ),
while others are nonstoichiometric
interstitial compounds (e.g. Pd 4 Se,
PdSe 2 ).
selenium Symbol Se. A metalloid
element belonging to group 16 (formerly 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, including grey, red, and black selenium. It
occurs in sulphide ores of other metals and is obtained as a by-product
(e.g. from the anode sludge in electrolytic reÜning). The element is a
semiconductor; the grey allotrope is
light-sensitive and is used in photocells, xerography, and similar applications. Chemically, it resembles
sulphur, and forms compounds with
selenium in the +2, +4, and +6 oxidasecondary-ion mass spectrometry
476
s
www.AzShimi.ir www.AzShimi.com