High Resolution Electron Energy Loss Spectroscopy - Overview of HREELS

Overview of HREELS

In general electron energy loss spectroscopy is based on the energy losses of electrons when inelastically scattered on matter. An incident beam of electrons with a known energy (E_i) is scattered on a sample. The scattering of these electrons can excite the electronic structure of the sample. If this is the case the scattered electron loses the specific energy (ΔE) needed to cause the excitation. Those scattering processes are called inelastic. It may be easiest to imagine that the energy loss is for example due to an excitation of an electron from an atomic K-shell to the M-shell. This energy for this excitation is taken away from the electrons kinetic energy. Then the energies of the scattered electrons (E_s) are measured and the energy loss can be calculated. From the measured data an intensity versus energy loss diagram is established. In the case of scattering on phonons the so called energy loss can also be a gain of energy (see: Raman spectroscopy). These energy losses allow, using comparison to other experiments, or theory, to draw conclusion about surface properties of the sample.

Excitations of the surface structure are usually very low energetic ranging from 10−3 eV to 10 eV. In spectra electrons with only small energy losses, like also Raman scattering, the interesting features are all located very close together and especially near to the very strong elastic scattering peak. Hence the used spectrometers require a high resolution. Therefore this regime of EELS is called High Resolution EELS. In this context resolution shall be defined as the energy difference in which two features in a spectrum are just distinguishable divided by the mean energy of those features:

In the case of EELS the first thing to think of in order to achieve high resolution is using incident electrons of a very precisely defined energy and a high quality analyzer. Further high resolution is only possible when the energies of the incident electrons are not far bigger than the energy losses. For HREELS the energy of the incident electrons is therefore mostly significantly smaller than 102 eV.

Considering that 102 eV electrons have a mean free path of around 1 nm (corresponds to a few monolayers), which is decreasing with lower energies, this automatically implies that HREELS is a surface sensitive technique. This is the reason why HREELS must be measured in reflection and has to be implemented in ultra high vacuum (UHV). In contrast to e.g. Core Level EELS which operates at very high energies and can therefore also be found in transmission electron microscopes (TEM).

In HREELS not only the electron energy loss can be measured, often the angular distribution, of electrons of a certain energy loss, in reference to the specular direction gives interesting insight to the structures on the surface.