Figure 119: Principle of optical measurementFigure 120: Principle of reflection electron energy loss spectroscopytroscopy). If x-rays are used as radiation source it is called XPS (x-ray photoemission spectroscopy).
The amount of electrons at a certain energy gives information about the density of states of the ma-
terial. As we already know, there are occupied states in the atom up to the fermi level. The incoming
photon can hit an electron and puts it from the bounded level to the vacuum level. So the pattern
which comes out of the energy analyzer is the same as the pattern of the density of states. Fig. 122
shows the measurement for silver and we can see that there are no occupied states after the fermi
level.
14.2.5 Raman spectroscopy
The principle of Raman spectroscopy is the inelastic scattering of light. Normally it is done with visible
or infrared light. If the light is shined on the sample most of the light is scattered elastically, which
means that the frequency and wavelength of the outgoing beam is the same as for the incoming beam.
Some of the light is scattered inelastically, so a part of the energy is given to a phonon, a plasmon,
a magnon, a polariton or to an exciton. Because of the conservation of charge, it is not allowed to
emit an electron or a polaron. The outgoing light will have a lower frequency and a longer wavelength
and this is called a stokes process in Raman spectroscopy. It could also happen that the incoming
light absorbs a phonon or something like that and so the outcoming light has a higher frequency and
a lower wavelength. This is called an anti-stokes process. This two situations are shown in fig. 123.