which a high negative voltage is applied to a metal tip placed in an evacuated vessel some distance from a
glass screen with a Ûuorescent coating. The tip produces electrons by
Üeld emission, i.e. the emission of
electrons from an unheated sharp
metal part as a result of a high electric Üeld. The emitted electrons form
an enlarged pattern on the Ûuorescent screen, related to the individual
exposed planes of atoms. As the resolution of the instrument is limited by
the vibrations of the metal atoms, it
is helpful to cool the tip in liquid helium. Although the individual atoms
forming the point are not displayed,
individual adsorbed atoms of other
substances can be, and their activity
is observable.
Üeld-ionization microscope
(Üeld-ion microscope; FIM) A type of
electron microscope that is similar in
principle to the *Üeld-emission
microscope, except that a high positive voltage is applied to the metal
tip, which is surrounded by lowpressure gas (usually helium) rather
than a vacuum. The image is formed
in this case by Üeld ionization: ionization at the surface of an unheated
solid as a result of a strong electric
Üeld creating positive ions by electron transfer from surrounding
atoms or molecules. The image is
formed by ions striking the Ûuorescent screen. Individual atoms on the
surface of the tip can be resolved
and, in certain cases, adsorbed atoms
may be detected. See also atom-probe
field-ion microscopy.
Üller A solid inert material added to
a synthetic resin or rubber, either to
change its physical properties or simply to dilute it for economy.
Ülm badge A lapel badge containing masked photographic Ülm worn
by personnel who could be exposed
to ionizing radiation. The Ülm is developed to indicate the extent that
the wearer has been exposed to
harmful radiation.
Ülter A device for separating solid
particles from a liquid or gas. The
simplest laboratory Ülter for liquids
is a funnel in which a cone of paper
(Ülter paper) is placed. Special containers with a porous base of sintered
glass are also used. See also gooch
crucible.
Ülter pump A simple laboratory
vacuum pump in which air is removed from a system by a jet of
water forced through a narrow nozzle. The lowest pressure possible is
the vapour pressure of water.
Ültrate The clear liquid obtained by
Ültration.
Ültration The process of separating
solid particles using a Ülter. In vacuum Ültration, the liquid is drawn
through the Ülter by a vacuum
pump. UltraÜltration is Ültration
under pressure.
FIM See field-ionization microscope.
Üne chemicals Chemicals produced industrially in relatively small
quantities and with a high purity;
e.g. dyes and drugs.
Üneness of gold A measure of the
purity of a gold alloy, deÜned as the
parts of gold in 1000 parts of the
alloy by mass. Gold with a Üneness of
750 contains 75% gold, i.e. 18 *carat
gold.
Üne structure Closely spaced spectral lines arising from transitions between energy levels that are split by
the vibrational or rotational motion
of a molecule or by electron spin.
They are visible only at high resolution. HyperÜne structure, visible only
at very high resolution, results from
the inÛuence of the atomic nucleus
225
fine structure
f
www.AzShimi.ir www.AzShimi.com
glass screen with a Ûuorescent coating. The tip produces electrons by
Üeld emission, i.e. the emission of
electrons from an unheated sharp
metal part as a result of a high electric Üeld. The emitted electrons form
an enlarged pattern on the Ûuorescent screen, related to the individual
exposed planes of atoms. As the resolution of the instrument is limited by
the vibrations of the metal atoms, it
is helpful to cool the tip in liquid helium. Although the individual atoms
forming the point are not displayed,
individual adsorbed atoms of other
substances can be, and their activity
is observable.
Üeld-ionization microscope
(Üeld-ion microscope; FIM) A type of
electron microscope that is similar in
principle to the *Üeld-emission
microscope, except that a high positive voltage is applied to the metal
tip, which is surrounded by lowpressure gas (usually helium) rather
than a vacuum. The image is formed
in this case by Üeld ionization: ionization at the surface of an unheated
solid as a result of a strong electric
Üeld creating positive ions by electron transfer from surrounding
atoms or molecules. The image is
formed by ions striking the Ûuorescent screen. Individual atoms on the
surface of the tip can be resolved
and, in certain cases, adsorbed atoms
may be detected. See also atom-probe
field-ion microscopy.
Üller A solid inert material added to
a synthetic resin or rubber, either to
change its physical properties or simply to dilute it for economy.
Ülm badge A lapel badge containing masked photographic Ülm worn
by personnel who could be exposed
to ionizing radiation. The Ülm is developed to indicate the extent that
the wearer has been exposed to
harmful radiation.
Ülter A device for separating solid
particles from a liquid or gas. The
simplest laboratory Ülter for liquids
is a funnel in which a cone of paper
(Ülter paper) is placed. Special containers with a porous base of sintered
glass are also used. See also gooch
crucible.
Ülter pump A simple laboratory
vacuum pump in which air is removed from a system by a jet of
water forced through a narrow nozzle. The lowest pressure possible is
the vapour pressure of water.
Ültrate The clear liquid obtained by
Ültration.
Ültration The process of separating
solid particles using a Ülter. In vacuum Ültration, the liquid is drawn
through the Ülter by a vacuum
pump. UltraÜltration is Ültration
under pressure.
FIM See field-ionization microscope.
Üne chemicals Chemicals produced industrially in relatively small
quantities and with a high purity;
e.g. dyes and drugs.
Üneness of gold A measure of the
purity of a gold alloy, deÜned as the
parts of gold in 1000 parts of the
alloy by mass. Gold with a Üneness of
750 contains 75% gold, i.e. 18 *carat
gold.
Üne structure Closely spaced spectral lines arising from transitions between energy levels that are split by
the vibrational or rotational motion
of a molecule or by electron spin.
They are visible only at high resolution. HyperÜne structure, visible only
at very high resolution, results from
the inÛuence of the atomic nucleus
225
fine structure
f
www.AzShimi.ir www.AzShimi.com