Figure 118: Solution of the energy, when frenkel excitons move through a perfect crystalis important that there are well defined homogenous layers in the crystal, so that one would be able
to know of which layers the peaks are coming from. So ellipsometry is very surface sensitive because
the light gets only a few atomic layers into the crystal before it will be sent out again.
The advantage of ellipsometry is that the ratio of the polarizations is independent of fluctuations of
the air or of the light source.
14.2.3 Reflection electron energy loss spectroscopy
Instead of sending light to the crystal, electrons are used as incoming beam and the scattered electrons
are measured. The fast moving electrons are creating a time dependent electric field and this field
polarizes the material. The oscillating dipoles of the polarized sample are sending out radiation. If
the polarization of the material moves out of phase with the electric field of the electrons, energy will
be lost. This means that the energy of the reflected electrons must be less than the energy of the
incoming ones. It is possible to calculate the dielectric constant with the informations collected by
this experiment.
14.2.4 Photo emission spectroscopy
At this type of spectroscopy photons with high energy are shot against the sample and electrons are
knocked out. When this is done with ultraviolet light, it is called UPS (ultraviolet photoemission spec-