High resolution electron energy loss spectroscopy (HREELS) is a tool used in surface science. The inelastic scattering of electrons from surfaces is utilized to study electronic excitations or vibrational modes of the surface of a material or of molecules adsorbed to a surface. In contrast to other electron energy loss spectroscopies (EELS), HREELS deals with small energy losses in the range of 10−3 eV to 1 eV. It plays an important role in the investigation of surface structure, catalysis, dispersion of surface phonons and the monitoring of epitaxial growth.
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 (Ei) 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. The energy for this excitation is taken away from the electron's kinetic energy. The energies of the scattered electrons (Es) 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 by phonons the so-called energy loss can also be a gain of energy (similar to anti-Stokes Raman spectroscopy). These energy losses allow, using comparison to other experiments or theory, one to draw conclusions about surface properties of a sample. Excitations of the surface structure are usually very low energy, ranging from 10−3 eV to 10 eV. In HREELS 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 EELS spectrometers require a high energy 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: Δ E / E {\displaystyle \Delta E/E}
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 decreases with lower energies, this automatically implies that HREELS is a surface sensitive technique. This is the reason why HREELS must be measured in reflection mode and must be implemented in ultra high vacuum (UHV). This is in contrast to Core Level EELS which operates at very high energies and can therefore also be found in transmission electron microscopes (TEM). Instrumental developments have also enabled vibrational spectroscopy to be performed in 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 a surface.
Physics of HREELS As mentioned above HREELS involves an inelastic scattering process on a surface. For those processes the conservation of energy as well as the conservation of momentum's projection parallel to the surface hold:
E are energies, k and q are wave vectors and G denotes a reciprocal lattice vector. One should mention at this point that for non perfect surfaces G is not in any case a well defined quantum number, what has to be considered when using the second relation. Variables subscripted with i denote values of incident electrons those subscripted with s values of scattered electrons. "||" denotes parallel to the surface. For the description of the inelastic scattering processes due to the excitation of vibrational modes of adsorbates different approaches exist. The simplest approach distinguishes between regimes of small and large scattering angles:
Dipole scattering
… excerpt ends here. Continue reading the full article.
