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Surface Science Techniques Title

EELS - Electron Energy Loss Spectroscopy
HREELS - High Resolution Electron Energy Loss Spectroscopy
SPEELS - Spin Polarised Electron Energy Loss Spectroscopy


EELS can be applied with several techniques, but always involves the bombardment of a sample with a monoenergetic beam of electrons. The technique is frequently used in association with Transmission Electron Microscopy (TEM) or Scanning TEM (STEM). The electrons impinging on the sample may lose energy by a variety of mechanisms. These losses can reveal the composition of the sample in TEM or STEM. Plasmon losses are a frequent cause of energy loss. Plasmons are collective excitations of the electron gas in the material and are typically several electron Volts in magnitude. Phonon losses can also occur, which are much smaller, and the energy spread of the monoenergetic beam must be particularly small to detect such losses. Phonons are quantized sound waves within the solid.

In TEM or STEM, the losses predominantly occur in the bulk of the sample, as the beam travels through the thin specimen to the EELS detector the other side. In surface science techniques, the electron beam is usually reflected of the surface resulting in a sharp peak corresponding to elastically scattered electrons with a number of peaks at lower energy which correspond to plasmon or other excitations.

By examining energy losses at high resolution (about 30 meV), as in HREELS, data concerning the vibrations of molecules on surfaces can be determined. Hence HREELS provides complementary information to RAIRS.

Another technique that is worthy of mention is spin polarised EELS or SPEELS which can provide information on phenomena such as magnetic coupling and exchange excitation processes.



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