Electron - Sample Interaction
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").
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Electron - Sample Atom Interaction |
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Interaction |
|
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 |
