Parallel-detection EELS

Theliterature contains many publications on self-made magnetic EEL spectrometers [2.182-2.196], and such systems have also been commercially available for many years. Several companies have developed such spectrometers, but the 90° magnetic prisms of the firm Gatan (parallel-detection EELS model 666 and the new Enfma model) are the only ones recently offered for TEM/STEM. Because they can easily be attached to a transmission microscope below its camera chamber, they are widely used. 

Two types of detection system are also available for EELS. Parallel detection spectrometers are fitted below the viewing chamber in a transmission electron microscope (TEM) or scanning transmission electron microscope (STEM). These spectrometers allow the detection of EELS spectra, which are plots... 

Figure Bl.25.12 illustrates the two scattering modes for a hypothetical adsorption system consisting of an atom on a metal [3]. The stretch vibration of the atom perpendicular to the surface is accompanied by a change m dipole moment the bending mode parallel to the surface is not. As explained above, the EELS spectrum of electrons scattered in the specular direction detects only the dipole-active vibration. The more isotropically scattered electrons, however, undergo impact scattering and excite both vibrational modes. Note that the comparison of EELS spectra recorded in specular and off-specular direction yields infomiation about the orientation of an adsorbed molecule.

Some crystallites will dissociate in the beam while others tend to agglomerate (14). The mass of the crystallite, support-metal interaction, chemical environment, oxidation state of the metal, etc., all have an influence on how the crystallite and electron beam interact. In order to formulate a correlation of these variables with crystallite reactivity with the beam, the crystallite site chemistry is required. This is difficult if not impossible to do because the site chemistry is altered during microscopic examination. With parallel EELS detection the time may be sufficiently reduced that useful chemical information can be obtained and correlations of the type previously described can be made. 

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