If we want to learn something about a system, a general experimental
approach is a scattering technique: we shoot some particles in a
well-prepared state on the
target and look at particles coming out of the target (which do not have to be
the same). In surface science the most basic questions we want to
solve with this approach are for example: Is the surface clean?
Which elements are on the
surface? And in which chemical compound? What is the exact geometric
structure of the surface?
The most common particles to scatter from surfaces are electrons,
ions, atoms and photons both as probe and response particles. An
important issue is the surface sensitivity of an experiment.
In general, it is high if we choose particles
which have a small mean free path in the solid because this means that
the detected particles must originate near the surface. The opposite
is true, for example, when the scattering of light by a surface is
investigated (reflectivity and
change of polarization). The
photons will penetrate relatively deeply into the crystal. The amount of
photons scattered at or near the surface will be very small. Hence,
light scattering is not a good tool to study surfaces.
In some cases we can increase the surface sensitivity by choosing an
experimental set-up where we use a very grazing angle of incidence or
emission. In this way the particles travel a long way close to the
surface, even if their mean free path is relatively long.
Very many surface science techniques are based on electrons as a
probe. Electrons have very useful properties: they are, at
certain energies, very surface sensitive. Electrons in this energy
range carry also enough momentum to explore the whole surface Brilloin
zone of a material (in contrast to light), they also carry a spin and
they are easy to generate and to handle.
The extensive use of electrons in surface sciences justifies a
lecture
explaining the physics of electron-solid interaction in some more
detail. Along with this, we will start to learn about some
electron-based analytical techniques.
A technique which is of particular interest in this lecture is
Electron Energy Loss Spectroscopy (EELS)
where a beam of monochromatic
electrons is scattered from the surface. A sketch of this experiment
is given in Fig. .
Figure: An EELS experiment. The momentum transfer parallel to the
surface is determined by the electron energy and the scattering
geometry.
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