terials, such as gallium arsenide, are
suitable for this purpose. A junction
between p-type and n-type semicon-
ductors can be used with the light
passing along the plane of the junc-
tion. 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 calculationsA
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 *ab-
initio calculations. As computing
power has increased, the properties
of large molecules can be calculated
by ab-initio calculations, therefore
the use of semi-empirical calcula-
tions has steadily declined,Ürst for
small molecules and more recently
for large molecules.
semimetal See metalloid.
semipermeable membrane A
membrane that is permeable to mol-
ecules of the solvent but not the
solute in *osmosis. Semipermeable
membranes can be made by support-
ing aÜlm of material (e.g. cellulose)
on a wire gauze or porous pot.
semipolar bond See chemical
bond.
SemtexA nitrogen-based stable
odourless plastic *explosive.
septivalent(heptavalent)Having a
valency of seven.
sequence rule See cip system.
sequestrationThe 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 solu-
tion (e.g. the sequestration of Ca2+
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.serineSee amino acid.
serpentineAny of a group of hy-
drous magnesium silicate minerals
with the general composition
Mg 3 Si 2 O 5 (OH) 4. Serpentine is mono-
clinic and occurs in two main forms:
chrysotile, which isÜbrous and the
chief source of *asbestos; and antig-
orite, which occurs as platy masses. It
is generally green or white with a
mottled appearance, sometimes re-
sembling a snakeskin – hence the
name. It is formed through the meta-
morphic alteration of ultrabasic
rocks rich in olivine, pyroxene, and
amphibole. Serpentiniteis 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 ex-
ample, a sesquioxide has the formula
M 2 O 3.
sesquiterpeneSee terpenes.SEXAFS(surface-extended X-ray ab-
sorptionÜne-structure spectroscopy)
A technique that makes use of the os-
cillations in X-ray absorbance found
in the high-frequency side of the ab-
sorption edge to investigate the
structure of surfaces. The physical
principle behind SEXAFS is quantum-
mechanical 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 prob-
ability of appearing, while in the case479 SEXAFS
s