optical microscopes the resolution is
limited by the wavelength of the
light. High-energy electrons, however, can be associated with a considerably shorter wavelength than light;
for example, electrons accelerated to
an energy of 10
5 electronvolts have a
wavelength of 0.004 nanometre (see
de broglie wavelength) enabling a
resolution of 0.2–0.5 nm to be
achieved. The transmission electron
microscope has an electron beam,
sharply focused by electron lenses,
passing through a very thin metallized specimen (less than 50 nanometres thick) onto a Ûuorescent screen,
where a visual image is formed. This
image can be photographed. The
scanning electron microscope can be
used with thicker specimens and
forms a perspective image, although
the resolution and magniÜcation are
lower. In this type of instrument a
beam of primary electrons scans
the specimen and those that are
reÛected, together with any secondary electrons emitted, are collected. This current is used to
modulate a separate electron beam
in a TV monitor, which scans the
screen at the same frequency, consequently building up a picture of the
specimen. The resolution is limited
to about 10–20 nm.
electron-nuclear double resonance See endor.
electron paramagnetic resonance (EPR) A spectroscopic
method of locating electrons within
the molecules of a paramagnetic substance (see magnetism) in order to
provide information regarding its
bonds and structure. The spin of an
unpaired electron is associated with
a magnetic moment that is able to
align itself in one of two ways with
an applied external magnetic Üeld.
These two alignments correspond to
different energy levels, with a statistical probability, at normal temperatures, that there will be slightly more
in the lower state than in the higher.
By applying microwave radiation to
the sample a transition to the higher
state can be achieved. The precise energy difference between the two
states of an electron depends on the
surrounding electrons in the atom or
molecule. In this way the position of
unpaired electrons can be investigated. The technique is used particularly in studying free radicals and
paramagnetic substances such as inorganic complexes. It is also called
electron-spin resonance (ESR). See also
nuclear magnetic resonance;
endor.
electron probe microanalysis
(EPM) A method of analysing a very
small quantity of a substance (as little as 10
–13 gram). The method consists of directing a very Ünely focused
beam of electrons on to the sample
to produce the characteristic X-ray
spectrum of the elements present. It
can be used quantitatively for elements with atomic numbers in excess of 11.
electron-spin resonance See electron paramagnetic resonance.
electron-transfer reaction A
chemical reaction that involves the
transfer, addition, or removal of electrons. Electron-transfer reactions
often involve complexes of transition
metals. In such complexes one general mechanism for electron transfer
is the inner-sphere mechanism, in
which two complexes form an intermediate, with ligand bridges enabling electrons to be transferred
from one complex to another complex. The other main mechanism is
the outer-sphere mechanism, in
which two complexes retain all their
ligands, with electrons passing from
one complex to the other.
The rates of electron-transfer reac199
electron-transfer reaction
e
www.AzShimi.ir www.AzShimi.com
limited by the wavelength of the
light. High-energy electrons, however, can be associated with a considerably shorter wavelength than light;
for example, electrons accelerated to
an energy of 10
5 electronvolts have a
wavelength of 0.004 nanometre (see
de broglie wavelength) enabling a
resolution of 0.2–0.5 nm to be
achieved. The transmission electron
microscope has an electron beam,
sharply focused by electron lenses,
passing through a very thin metallized specimen (less than 50 nanometres thick) onto a Ûuorescent screen,
where a visual image is formed. This
image can be photographed. The
scanning electron microscope can be
used with thicker specimens and
forms a perspective image, although
the resolution and magniÜcation are
lower. In this type of instrument a
beam of primary electrons scans
the specimen and those that are
reÛected, together with any secondary electrons emitted, are collected. This current is used to
modulate a separate electron beam
in a TV monitor, which scans the
screen at the same frequency, consequently building up a picture of the
specimen. The resolution is limited
to about 10–20 nm.
electron-nuclear double resonance See endor.
electron paramagnetic resonance (EPR) A spectroscopic
method of locating electrons within
the molecules of a paramagnetic substance (see magnetism) in order to
provide information regarding its
bonds and structure. The spin of an
unpaired electron is associated with
a magnetic moment that is able to
align itself in one of two ways with
an applied external magnetic Üeld.
These two alignments correspond to
different energy levels, with a statistical probability, at normal temperatures, that there will be slightly more
in the lower state than in the higher.
By applying microwave radiation to
the sample a transition to the higher
state can be achieved. The precise energy difference between the two
states of an electron depends on the
surrounding electrons in the atom or
molecule. In this way the position of
unpaired electrons can be investigated. The technique is used particularly in studying free radicals and
paramagnetic substances such as inorganic complexes. It is also called
electron-spin resonance (ESR). See also
nuclear magnetic resonance;
endor.
electron probe microanalysis
(EPM) A method of analysing a very
small quantity of a substance (as little as 10
–13 gram). The method consists of directing a very Ünely focused
beam of electrons on to the sample
to produce the characteristic X-ray
spectrum of the elements present. It
can be used quantitatively for elements with atomic numbers in excess of 11.
electron-spin resonance See electron paramagnetic resonance.
electron-transfer reaction A
chemical reaction that involves the
transfer, addition, or removal of electrons. Electron-transfer reactions
often involve complexes of transition
metals. In such complexes one general mechanism for electron transfer
is the inner-sphere mechanism, in
which two complexes form an intermediate, with ligand bridges enabling electrons to be transferred
from one complex to another complex. The other main mechanism is
the outer-sphere mechanism, in
which two complexes retain all their
ligands, with electrons passing from
one complex to the other.
The rates of electron-transfer reac199
electron-transfer reaction
e
www.AzShimi.ir www.AzShimi.com