Electron energy loss spectroscopy

Electron energy loss spectroscopy is usually abbreviated in German with the acronym EELS ( English electron energy loss spectroscopy ) and in older literature it is also called EEVA ( electron energy loss analysis ). It is mainly used in 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 investigation of solid surfaces (see also vibration spectroscopy , electron spectroscopy ).

functionality

With this type of spectroscopy, the spectrum of the energy loss of monoenergetic (“ monochromatic ”) electrons is determined after interaction with a sample. 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 this primary energy distribution naturally also determines the achievable spectral resolution of the method.

The primary electrons interact via their electric field with the charged particles in the sample ( protons and electrons combined in atomic nuclei with neutrons ). Since the atomic nuclei are much more massive than individual electrons, the energy transfer from the primary electrons to the nuclei is negligible (so-called elastic and quasi-elastic scattering). It is different with the interaction with the solid-state electrons. Noticeable energy losses can occur here (inelastic scattering). The solid electrons as fermions are now not allowed to absorb any energy. The energetic states and transitions between them allowed for them are given by the band structure or, to a good approximation, for lower-lying energy levels by the atomic bond states . From this follows a characteristic probability distribution for energy transfers: the energy loss spectrum, which is determined in the EELS experiment.

Usually, EELS refers to the application of the method in the transmission electron microscope . The primary energy is from a few 10 keV to a few 100 keV (1 keV = 1000  eV ), with a distribution range of mostly 0.8 eV to about 2.5 eV. When using field emission cathodes, minimum widths of around 0.35 eV are achieved; even smaller energy widths require the use of energy-filtered electron sources, so-called monochromators. The investigated energy losses range from about 1 eV to several 1000 eV. ${\displaystyle E_{0}}$

The probability of an energy loss with the energy after the inelastic interaction with scattering at a certain angle is determined by means of the inelastic scattering cross section (also called inelastic effective cross section ) ${\displaystyle \Delta E=E_{0}-E_{1}}$ ${\displaystyle E_{1}}$

${\displaystyle {\frac {\mathrm {d} ^{2}\sigma }{\mathrm {d} \Delta E\mathrm {d} {\vec {\Omega }}}}\sim {\frac {1}{q^{2}}}\mathrm {Im} (-{\frac {1}{\epsilon (\Delta E,{\vec {q}})}})}$

expressed. Here is the difference in wave vectors of the scattered electron before and after the scattering. is the solid angle element . The occurrence of the dielectric function at this point results from the fact that the electrons cause polarization, and the dielectric function precisely describes the polarizability of materials. ${\displaystyle {\vec {q}}}$ ${\displaystyle \mathrm {d} {\vec {\Omega }}}$ ${\displaystyle \epsilon }$

variants

A modification and refinement of the method provides the high-resolution electron energy loss spectroscopy (HREELS, abbr. For English. High-resolution electron energy loss spectroscopy ), which the for vibrational spectroscopy considered important area to 15 to 600 meV. This method usually works with significantly lower primary electron energies than with normal energy loss spectroscopy, and special spectrometers are used. The low primary electron energy generally does not allow measurements to be taken in transmission. Rather, HREELS is a method of surface analysis .

See also

• Energy Filtered Transmission Electron Microscopy

swell

• P. Schattschneider : Fundamentals in Inelastic Scattering. Springer, Vienna et al. 1986, ISBN 3-211-81937-1 .

literature

• Ray F. Egerton: Electron Energy Loss Spectroscopy in the Electron Microscope . 2nd Edition. Plenum Press, New York 1996, ISBN 0-306-45223-5 .

Web links

• EELS - an introduction ( Memento from April 2, 2008 in the Internet Archive ), TU Vienna.
• A database of EELS and X-Ray excitation spectra

Individual evidence

1. ↑ For the derivation see z. BP Schattschneider: Fundamentals in Inelastic Scattering. Springer, Vienna / New York 1986, ISBN 3211819371 .