High resolution electron energy loss spectroscopy
High resolution electron energy loss spectroscopy (HREELS) is a tool used in surface science.
If this is the case the scattered electron loses the specific energy (ΔE) needed to cause the excitation.
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.
These energy losses allow, using comparison to other experiments or theory, one to draw conclusions about surface properties of a sample.
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).
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.
For the description of the inelastic scattering processes due to the excitation of vibrational modes of adsorbates different approaches exist.
It can be approached using the so-called dielectrical theory introduced by Lucas and Šunjić of which a quantum mechanical treatment was first presented by E. Evans and D.L.
[3] Alternatively there is a more unfamiliar model which only holds exactly for perfect conductors: A unit cell at the surface does not have a homogeneous surrounding, hence it is supposed to have an electrical dipole moment.
This dipole moment causes a long range electronic potential in the vacuum above the surface.
On this potential the incident electron can scatter inelastically which means it excites vibrations in the dipole structure.
When measuring the intensity of the electron energy loss peaks and comparing to other experimental results or to theoretical models it can also be determined whether a molecule is adsorbed normal to the surface or tilted by an angle.
In intermediate negative ion resonance the electron forms a compound state with an adsorbed molecule during the scattering process.
Where i denotes the initial and f the final vibrational energy level of the adsorbed molecule and pz the z component of its dipole moment.
As the dipole moment is something like charge times length, pz has the same symmetry properties as z, which is totally symmetric.
Note that it says nothing about the intensity for scattering or the displacement of the atoms of the adsorbate, but its total dipole moment is the operator in the matrix element.
When trying to gain information from selection rules, one must carefully consider whether a pure dipole or impact scattering region is investigated.
Another problem is that in cases of larger molecules often many vibrational modes are degenerate, which could again be resolved due to strong molecule-surface interactions.
But when carefully investigating it is mostly possible to get a very good picture of how the molecule adheres to the surface by analysis of normal dipole modes.
[citation needed] As the electrons used for HREELS are of low energy they do not only have a very short mean free path length in the sample materials but also under normal atmospheric conditions.
The spectrometer is in general a computer simulated design that optimizes the resolution while keeping an acceptable electron flux.
To enable measurements of angular distributions all those elements are mounted on a rotate able table with the axis cantered at the sample.Its negative charge causes the electron beam to broaden.
To make the analysis more accurate, especially to reduce the background of in the deflector scattered electrons often two analyzers are used, or additional apertures are added behind the analyzers as scattered electrons of the wrong energy normally leave the CHAs under large angles.
This has to be considered when doing the design of the setup as it is anyway difficult to keep different potentials, of repeller, lenses, screening elements, and the reflector, constant.
Because of the same reason the whole experiment, except the lenses which are normally made of coated copper, is designed in stainless antimagnetic steel and insulating parts are avoided wherever possible.
