|
Applications
of TEMs are not only in the field of high resolution imaging, where
the contrast is caused by elastic and inelastic scattering, but also
in microanalytics. In these modes interaction effects like emitted
X-rays, secondary electrons, cathodoluminiscence radiation or energy
losses of inelastic scattered electrons are analysed.
The most important component of contrast formation is elastic scattering
where electrons are scattered at the nuclei of the target atoms by
Coulomb forces.The kinetic energies and momenta of colliding particles
are unchanged in elastic scattering. For small scattering angles only
negligible amounts of energy and momentum are transferred from the
passing electrons to the nucleus. For larger scattering angles considerable
amounts of energy in the range of several tens of electronvolts can
be exchanged. This can lead to a loss of atoms by ejection ("knock-on").
|
Electron
- Sample Atom Interaction
|
|
Interaction
|
s
(cross section [cm2])
|
l
(mean free path)
|
Probability
per Electron
|
|
Ionisation
|
2.0
x 10-18
|
50
nm
|
4.5
x 10-3
|
|
Plasmon
Excitation
|
1.5
x 10-18
|
66
nm
|
3.3
x 10-3
|
|
X-ray
Emission (K)
|
2.5
x 10-20
|
4
µm
|
6.5
x 10-5
|
|
Ejection
of Atom
|
1.8
x 10-22
|
550
µm
|
4.0
x 10-7
|
|
Elastic
Scattering
|
1.1
x 10-18
|
100
nm
|
2.2
x 10-3
|
|
