Concluding remarks

We have seen that the minimum in the electron mean free path in Fig. is mainly determined by the excitations of plasmons. Below the edge for plasmon excitation, the most important loss mechanism is interband transitions. This raises the question about how universal the curve actually is. If we take, for example, a wide band semiconductor, then interband transitions below the gap energy will not be possible and this will increase the mean free path considerably. The fact that plasmon creation is the most important mechanism for inelastic scattering makes it clear why it is simple, in most cases, to distinguish elastically scattered (emitted) electrons from electrons which have been scattered by plasmons. The reason is that the plasma energies are rather large so that the loss peaks are far away from the elastic peak (see Fig. ). The most common way to measure the mean free path of electrons in a material is to evaporate a thin film of the material on a substrate. Then the intensity of characteristic Auger transitions   in the substrate is measured as a function of film thickness.

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