EELS

EELs is usually employed at high electron kinetic energies (up to 105 eV) with thin samples in the transmission mode. For the electrochemist, low kinetic energies ( 1 eV) in the reflection mode are of greatest interest as they can be used to study the vibrational spectra of adsorbates40. 

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Additional information can be obtained, if one calculates the smallest thickness difference Ad of sf eel - for instance the depth of a crack - which can be discerned on a radiograph whose granularity is just as high as the limiting value a, of the respective class of the standard EN 584-1. For this estimation the well known relation for the (optical) density difference AD (visible contrast) which results from a difference of thickness Ad in steel is used ... 

EELS Electron-energy-loss Incident electrons are Surface energy states ... 

CELS, EIS Characteristic-energy-loss spectroscopy, electron-impact spectroscopy [128] Same as EELS Same as EELS... 

HREELS High-resolution electron energy-loss spectroscopy [129, 130] Same as EELS Identification of adsorbed species through their vibrational energy spectrum... 

The 02-graphite system has been studied by means of quite a few techniques LEED, RHEED, EELS, and NEXAFS (see Ref. 101). It was concluded,... 

Electrons interact with solid surfaces by elastic and inelastic scattering, and these interactions are employed in electron spectroscopy. For example, electrons that elastically scatter will diffract from a single-crystal lattice. The diffraction pattern can be used as a means of stnictural detenuination, as in FEED. Electrons scatter inelastically by inducing electronic and vibrational excitations in the surface region. These losses fonu the basis of electron energy loss spectroscopy (EELS). An incident electron can also knock out an iimer-shell, or core, electron from an atom in the solid that will, in turn, initiate an Auger process. Electrons can also be used to induce stimulated desorption, as described in section Al.7.5.6. 

Vibrational spectroscopy provides detailed infonnation on both structure and dynamics of molecular species. Infrared (IR) and Raman spectroscopy are the most connnonly used methods, and will be covered in detail in this chapter. There exist other methods to obtain vibrational spectra, but those are somewhat more specialized and used less often. They are discussed in other chapters, and include inelastic neutron scattering (INS), helium atom scattering, electron energy loss spectroscopy (EELS), photoelectron spectroscopy, among others. 

Electron-impact energy-loss spectroscopy (EELS) differs from other electron spectroscopies in that it is possible to observe transitions to states below the first ionization edge electronic transitions to excited states of the neutral, vibrational and even rotational transitions can be observed. This is a consequence of the detected electrons not originating in the sample. Conversely, there is a problem when electron impact induces an ionizing transition. For each such event there are two outgoing electrons. To precisely account for the energy deposited in the target, the two electrons must be measured in coincidence. 

Joy D C 1986 The basic principles of EELS Principles of Analytical Electron Microscopy ed D C Joy, A D Romig Jr and J I Goldstein (New York Plenum)... 

For the parallel recording of EEL spectra in STEM, linear arrays of semiconductor detectors are used. Such detectors convert the incident electrons mto photons, using additional fluorescent coatings or scintillators in the very same way as the TEM detectors described above. 

For a detailed discussion on the analytical teclmiques exploiting the amplitude contrast of melastic images in ESI and image-EELS, see chapter B1.6 of this encyclopedia. One more recent but also very important aspect is the quantitative measurement of atomic concentrations in the sample. The work of Somlyo and colleagues [56]. Leapman and coworkers and Door and Gangler [59] introduce techniques to convert measured... 

Leapman R D, Hunt J A, Buchanan R A and Andrews S B 1993 Measurement of iow caicium concentrations in cryosectioned ceiis by paraiiei-EELS mapping Ultramicroscopy 49 225-34... 

EELS Electron energy loss spectroscopy The loss of energy of low-energy electrons due to excitation of lattice vibrations. Molecular vibrations, reaction mechanism... 

Wliile infrared and Raman speetroseopy are limited to vibrations in whieh a dipole moment or the moleeular polarizability ehanges, EELS deteets all vibrations. Two exeitation meehanisms play a role in EELS dipole... 

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.

Figure Bl.25.12. Excitation mechanisms in electron energy loss spectroscopy for a simple adsorbate system Dipole scattering excites only the vibration perpendicular to the surface (v ) in which a dipole moment nonnal to the surface changes the electron wave is reflected by the surface into the specular direction. Impact scattering excites also the bending mode v- in which the atom moves parallel to the surface electrons are scattered over a wide range of angles. The EELS spectra show the higlily intense elastic peak and the relatively weak loss peaks. Off-specular loss peaks are in general one to two orders of magnitude weaker than specular loss peaks.

A strong point of EELS is that it detects losses in a very broad energy range, which comprises the entire infrared regime and extends even to electronic transitions at several electron volts. EELS spectrometers have to satisfy a number of stringent requirements. First, the primary electrons should be monochromatic. Second,... 

Stern D A, Wellner E, Salaita G N, Laguren-Davidson L, Lu F, Batina N, Frank D G, Zapien D C, Walton N and Flubbard A T 1988 Adsorbed thiophenol and related oompounds studied at Pt(111) eleotrode by EELS, Auger-speotrosoopy, and oyolie voltammetry J. Am. Chem. Soc. 110 4885-93... 

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