terials, such as gallium arsenide, are
suitable for this purpose. A junction
between p-type and n-type semiconductors can be used with the light
passing along the plane of the junction. Mirrors for the laser action are
provided by the ends of the crystals.
Semiconductor lasers can be as small
as 1 mm in length.
semi-empirical calculations A
technique for calculating atomic and
molecular quantities in which the
values of integrals are found using
quantities derived from experiment
(such as ionization energies obtained
spectroscopically). Semi-empirical
calculations were formerly widely
used, particularly for large molecules
involving extensive computation to
calculate all the integrals using *abinitio calculations. As computing
power has increased, the properties
of large molecules can be calculated
by ab-initio calculations, therefore
the use of semi-empirical calculations has steadily declined, Ürst for
small molecules and more recently
for large molecules.
semimetal See metalloid.
semipermeable membrane A
membrane that is permeable to molecules of the solvent but not the
solute in *osmosis. Semipermeable
membranes can be made by supporting a Ülm of material (e.g. cellulose)
on a wire gauze or porous pot.
semipolar bond See chemical
bond.
Semtex A nitrogen-based stable
odourless plastic *explosive.
septivalent (heptavalent) Having a
valency of seven.
sequence rule See cip system.
sequestration The process of
forming coordination complexes of
an ion in solution. Sequestration
often involves the formation of
chelate complexes, and is used to
prevent the chemical effect of an ion
without removing it from the solution (e.g. the sequestration of Ca
2+
ions in water softening). It is also a
way of supplying ions in a protected
form, e.g. sequestered iron solutions
for plants in regions having alkaline
soil.
serine See amino acid.
serpentine Any of a group of hydrous magnesium silicate minerals
with the general composition
Mg 3 Si 2 O 5 (OH) 4 . Serpentine is monoclinic and occurs in two main forms:
chrysotile, which is Übrous and the
chief source of *asbestos; and antigorite, which occurs as platy masses. It
is generally green or white with a
mottled appearance, sometimes resembling a snakeskin – hence the
name. It is formed through the metamorphic alteration of ultrabasic
rocks rich in olivine, pyroxene, and
amphibole. Serpentinite is a rock
consisting mainly of serpentine; it is
used as an ornamental stone.
sesqui- PreÜx indicating a ratio of
2:3 in a chemical compound. For example, a sesquioxide has the formula
M 2 O 3 .
sesquiterpene See terpenes.
SEXAFS (surface-extended X-ray absorption Üne-structure spectroscopy)
A technique that makes use of the oscillations in X-ray absorbance found
in the high-frequency side of the absorption edge to investigate the
structure of surfaces. The physical
principle behind SEXAFS is quantummechanical interference between the
wave function of a photoelectron and
the wave function associated with
scattering by surrounding atoms. In
the case of constructive interference,
the photoelectron has a higher probability of appearing, while in the case
479
SEXAFS
s
www.AzShimi.ir www.AzShimi.com
suitable for this purpose. A junction
between p-type and n-type semiconductors can be used with the light
passing along the plane of the junction. Mirrors for the laser action are
provided by the ends of the crystals.
Semiconductor lasers can be as small
as 1 mm in length.
semi-empirical calculations A
technique for calculating atomic and
molecular quantities in which the
values of integrals are found using
quantities derived from experiment
(such as ionization energies obtained
spectroscopically). Semi-empirical
calculations were formerly widely
used, particularly for large molecules
involving extensive computation to
calculate all the integrals using *abinitio calculations. As computing
power has increased, the properties
of large molecules can be calculated
by ab-initio calculations, therefore
the use of semi-empirical calculations has steadily declined, Ürst for
small molecules and more recently
for large molecules.
semimetal See metalloid.
semipermeable membrane A
membrane that is permeable to molecules of the solvent but not the
solute in *osmosis. Semipermeable
membranes can be made by supporting a Ülm of material (e.g. cellulose)
on a wire gauze or porous pot.
semipolar bond See chemical
bond.
Semtex A nitrogen-based stable
odourless plastic *explosive.
septivalent (heptavalent) Having a
valency of seven.
sequence rule See cip system.
sequestration The process of
forming coordination complexes of
an ion in solution. Sequestration
often involves the formation of
chelate complexes, and is used to
prevent the chemical effect of an ion
without removing it from the solution (e.g. the sequestration of Ca
2+
ions in water softening). It is also a
way of supplying ions in a protected
form, e.g. sequestered iron solutions
for plants in regions having alkaline
soil.
serine See amino acid.
serpentine Any of a group of hydrous magnesium silicate minerals
with the general composition
Mg 3 Si 2 O 5 (OH) 4 . Serpentine is monoclinic and occurs in two main forms:
chrysotile, which is Übrous and the
chief source of *asbestos; and antigorite, which occurs as platy masses. It
is generally green or white with a
mottled appearance, sometimes resembling a snakeskin – hence the
name. It is formed through the metamorphic alteration of ultrabasic
rocks rich in olivine, pyroxene, and
amphibole. Serpentinite is a rock
consisting mainly of serpentine; it is
used as an ornamental stone.
sesqui- PreÜx indicating a ratio of
2:3 in a chemical compound. For example, a sesquioxide has the formula
M 2 O 3 .
sesquiterpene See terpenes.
SEXAFS (surface-extended X-ray absorption Üne-structure spectroscopy)
A technique that makes use of the oscillations in X-ray absorbance found
in the high-frequency side of the absorption edge to investigate the
structure of surfaces. The physical
principle behind SEXAFS is quantummechanical interference between the
wave function of a photoelectron and
the wave function associated with
scattering by surrounding atoms. In
the case of constructive interference,
the photoelectron has a higher probability of appearing, while in the case
479
SEXAFS
s
www.AzShimi.ir www.AzShimi.com