Electron energy loss spectroscopy

Electron energy loss spectroscopy is abbreviated in German usually known by the acronym EELS ( electron energy loss spectroscopy English ) and in older literature also known as EEVA ( electron energy loss analysis). It is used mainly in the analytical transmission electron microscopy for the stoichiometric and electronic characterization of inorganic and organic structures and, as a special form HREELS, in Surface Chemistry and Surface physics for the study of solid surfaces ( see also vibrational spectroscopy, electron spectroscopy ).

Operation

In this Spektroskopieart the spectrum of the electron energy loss is determined by interaction with a sample initially monoenergetic ( " monochromatic "). Monoenergetic here means in particular that the width of the energy distribution of the primary electrons should be small compared to the width of the measured spectrum, the width of the primary energy distribution, of course, also determines the achievable spectral resolution of the method.

The primary electrons interact via their electric field with the charged particles of the sample ( in atomic nuclei with neutrons protons and electrons combined ). Since the nuclei are much more massive than single electrons, the energy transfer from the primary electrons on the nuclei is negligible (so-called elastic and quasi-elastic scattering). The situation is different in the interaction with the solid-state electrons. Here there may be appreciable energy losses occur ( inelastic scattering). The solid-state electrons as fermions no any energy shots are now permitted. The permissible for her energetic states and transitions between them are determined by the band structure or a good approximation for lower-lying energy levels of the atomic bound states. This results in a probability distribution characteristic for energy transfers: the energy loss spectrum is determined in the EELS experiment.

Most is meant by transmission electron microscopy with EELS the application of the method. Primary energy is a few to a few 10 keV 100 keV ( 1 keV = 1000 eV), a distribution width of usually 0.8 eV to about 2.5 eV. With the use of field emission cathodes is achieved minimum width of about 0.35 eV, more lower energy widths require the use of energy- filtered electron sources, so-called monochromator. The investigated energy losses extending to some 1000 eV of about 1 eV.

The likelihood of a loss of energy to the energy after interaction with the inelastic scattering at a certain angle by means of the inelastic scattering cross-section (also known as inelastic cross section )

Expressed. Here is before the difference between the wave vectors of the scattered electron and after scattering. is the Raumwinkelement. The appearance of the dielectric function at this point due to the fact that the electrons cause polarization and the dielectric function of the polarizability of materials just described.

Variants

A modification and refinement of the method, the high-resolution electron energy loss spectroscopy ( HREELS, Abbreviation for. High-resolution electron energy loss spectroscopy ), which considered important for the vibrational spectroscopy range to 15 to 600 meV. In this method is usually carried out with a significantly lower energy than the primary electrons at the normal energy loss spectroscopy, and specific spectrometers are used. The low primary electron energy usually allowed not to measure in transmission. Rather, HREELS is a method of surface analysis.