optical microscopes the resolution is
limited by the wavelength of the
light. High-energy electrons, how-
ever, can be associated with a consid-
erably 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 metal-
lized specimen (less than 50 nanome-
tres thick) onto aÛuorescent 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 sec-
ondary electrons emitted, are col-
lected. This current is used to
modulate a separate electron beam
in a TV monitor, which scans the
screen at the same frequency, conse-
quently building up a picture of the
specimen. The resolution is limited
to about 10–20 nm.
electron-nuclear double reso-
nanceSee endor.
electron paramagnetic reso-
nance(EPR)A spectroscopic
method of locating electrons within
the molecules of a paramagnetic sub-
stance (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 magneticÜeld.
These two alignments correspond to
different energy levels, with a statis-
tical probability, at normal tempera-
tures, 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 en-
ergy 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 investi-
gated. The technique is used particu-
larly in studying free radicals and
paramagnetic substances such as in-
organic 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 lit-
tle as 10–13gram). The method con-
sists of directing a veryÜnely 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 el-
ements with atomic numbers in ex-
cess of 11.electron-spin resonance See elec-
tron paramagnetic resonance.
electron-transfer reaction A
chemical reaction that involves the
transfer, addition, or removal of elec-
trons. Electron-transfer reactions
often involve complexes of transition
metals. In such complexes one gen-
eral mechanism for electron transfer
is the inner-sphere mechanism, in
which two complexes form an inter-
mediate, with ligand bridges en-
abling electrons to be transferred
from one complex to another com-
plex. 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 reac-199 electron-transfer reaction
e