High resolution electron energy loss spectroscopy
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High resolution electron energy loss spectroscopy (HREELS) is a kind of surface vibrational spectroscopy. A fraction of the incident electrons lose energy by exciting atomic and molecular vibrations at the surface, leading to energy loss peaks that can be observed.
Since vibrational energies in molecules range between hundreds of an electron volt and a few tenths of an electron volt, the primary electron beam must be highly monochromatic with an energy spread less than 10 meV at a typical primary beam energy of between 1 and 10 eV.
HREELS has several vibrational excitation mechanisms.
[edit] Dipole scattering
This is a long range mechanism. The electric field of an incoming electron interacts with the changing electric field by molecular vibration only perpendicular to the surface (surface selection rule). The intensity distribution is sharply peaked in the specular direction.
[edit] Impact scattering
This is a short-range mechanism. The incoming electrons impact atoms and molecules at the surface, leading to vibrational excitation. This mechanism excites all vibrations (both parallel and perpendicular to the surface). Since the scattering direction is isotropic, the dipole-forbidden mode can be best observed in the off-specular direction so as to exclude the intensity by dipole scattering.
