The incoming electrons take note of the oscillating dipoles that are present on the surface. Most often these arise from the vibrational modes of the molecular adsorbates that are present. It is also aware of the chemisorption bonds to the surface. Furthermore, phonons in the surface of semiconducting substrates (not metallic) also interact with the electron beam.
Two scattering modes are observed, differentiated by the mechanism with which they scatter.
This is a long-range effect mediated by the Coulomb field whereby the incoming charged electron is influenced by a vibrating dipole at the surface.(as shown in below animator) Such an electron is scattered specularly with an energy loss characteristic of the energy it deposited in the vibrational mode. Hence, such information is exactly like an IR spectrum. And, as with RAIRS, the normal dipole selection rule still applies, that is that it is blind to dipoles which oscillate parallel to the surface and can only interact with dipoles perpendicular (normal) to the surface.
This is a short range scattering process from the ion core. The scattering is more isotropic (not in the specular direction, but rather everywhere) but the energy losses still reflect vibrational excitations in the adsorbate. The angular distribution of peaks around the specular direction can distinguish between peaks which result from different scattering modes (Dipole Scattering is dominant).
Schematic illustration of the operation of the normal dipole selection rule in HREELS. Image dipoles within the surface either enhance
(for normal orientation on the left) or negate (for parallel orientation on the right) the effect of dipoles in the vacuum above the surface
There is therefore a selection rule that favors normally dipoles.
It should be known that at large deflection angles, the dipole picture breaks down, and so does the selection rule which limits scattering only to modes with the oscillating dipole moment normal to the surface. Thus, the energy loss spectrum of electrons which scatter through large angles contains features from all possible vibrational modes of an adsorbed molecule; a comparison between the energy loss spectrum associated with large-angle scatterings and the near specular loss spectrum allows one to identify which modes have symmetry that produce an oscillating dipole moment normal to the surface. This places strong constraints on the nature of the adsorption site, and the molecular orientation in many cases.
Vibration of adsorbed Atom
Suface vibration are more than adsorbed molecular vibration , when atoms adsorbed on the surface, new vibration will occur.