it is supposed to have an electrical dipole moment. When a molecule is adsorbed to the surface there can be an additional dipole moment and the total dipole moment P is present. 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. The dipole moment can then be written as . When the adsorbate sticks to a metal surface, imaginary dipoles occur as shown in the figure on the right. Hence for an adsorbed dipole normal to the surface the dipole moment "seen" from the vacuum doubles. Whereas the dipole moment of a parallel to the surface adsorbed dipole vanishes. Hence an incident electron can excite the adsorbed dipole only when it is adsorbed normal to the surface and the vibrational mode can be detected in the energy loss spectrum. If the dipole is adsorbed parallel then no energy losses will be detected and the vibrational modes of the dipole are missing in the energy loss spectrum. 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. The dielectric model also holds when the material on which the molecule adsorbs is not a metal. The picture shown above is then the limit for where denotes the relative dielectrical constant. As the incident electron in this model is scattered in the region above the surface, it does not directly impact the surface and because the amount of momentum transferred is small the scattering is mostly in the specular direction. Impact scattering Impact scattering is the regime which deals with electrons that are scattered further away from the specular direction. In those cases no macroscopic theory exists and a microscopic theory like, quantum mechanical dispersion theory, has to be applied. Symmetry considerations then also result in certain selection rules (it is also assumed that the energy loss in the inelastic scattering process is negligible): When the scattering plane is a plane of reflection symmetry then the scattering amplitude for every ks in the scattering plane vanishes. When the plane perpendicular to the surface and the scattering plane is a plane of reflection symmetry and time reversal symmetry holds then the