netic properties. Examples are LaSiP 3 ,
K 6 Bi 2 Sn 23 , and Yb 14 MnSb 11. Com-
pounds of this type are known as
pnictides.
Pockels cellAn electro-optical de-
vice used to produce population in-
version in lasers and a pulse of
radiation. In a Pockels cell, crystals of
ammonium dihydrogenphosphate
are used to convert plane-polarized
light to circularly polarized light
when a potential difference is ap-
plied. A Pockels cell can be made
part of a laser cavity. When this hap-
pens, a change of polarization occurs
when light is reÛected from a mirror
and the light that is polarized in one
plane is converted into light polar-
ized in the perpendicular plane. The
effect is that the reÛected light does
not stimulate emission. When the
Pockels cell is turned off the polariza-
tion does not occur and the energy in
the cavity can be released in the
form of a pulse of stimulated radia-
tion. See also laser.point group A group of symmetry
elements that leave a point un-
changed, used in classifying mol-
ecules. Compare space group.
poiseA *c.g.s. unit of viscosity
equal to the tangential force in dynes
per square centimetre required to
maintain a difference in velocity of
one centimetre per second between
two parallel planes of aÛuid sepa-
rated by one centimetre. 1 poise is
equal to 10–1Nsm–2.
poison1.Any substance that is in-
jurious to the health of a living or-
ganism. 2.A substance that prevents
the activity of a catalyst. 3.A sub-
stance that absorbs neutrons in a nu-
clear reactor and therefore slows
down the reaction. It may be added
intentionally for this purpose or may
be formed as aÜssion product and
need to be periodically removed.polar compoundA compound
that is either ionic (e.g. sodium chlo-
ride) or that has molecules with a
large permanent dipole moment (e.g.
water).
polarimeter(polariscope)An in-
strument used to determine the
angle through which the plane of po-
larization of plane-polarized light is
rotated on passing through an opti-
cally active substance. Essentially, a
polarimeter consists of a light source,
a polarizer(e.g. a sheet of Polaroid)
for producing plane-polarized light, a
transparent cell containing the sam-
ple, and an analyser. The analyser is
a polarizing material that can be ro-
tated. Light from the source is plane-
polarized by the polarizer and passes
through the sample, then through
the analyser into the eye or onto a
light-detector. The angle of polariza-
tion is determined by rotating the
analyser until the maximum trans-
mission of light occurs. The angle of
rotation is read off a scale. Simple
portable polarimeters are used for es-
timating the concentrations of sugar
solutions in confectionary manufac-
ture.polariscope(polarimeter)A device
used to study optically active sub-
stances (see optical activity). The
simplest type of instrument consists
of a light source, collimator, polar-
izer, and analyser. The specimen
is placed between polarizer and
analyser, so that any rotation of the
plane of polarization of the light can
be assessed by turning the analyser.polarizabilitySymbol α. A meas-
ure of the response of a molecule to
an external electricÜeld. When a
molecule is placed in an external
electricÜeld, the displacement of
electric charge induces a dipole in
the molecule. If the electricÜeld
strength is denoted Eand the electri-
cal dipole moment induced by thisPockels cell 424p