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Electron Energy Loss Spectroscopy (EELS)

Description

EELS is part of the analytical techniques known as Vibrational Spectroscopies. It is based on scattering of low-energy electrons from solid surfaces to measure vibrational spectra of surface species. It can be considered as the electron analogue of Raman spectroscopy. In order to avoid confusion with other electron scattering spectroscopies it is often referred as HREELS, High-Resolution Electron Energy Loss Spectroscopy.

 
 

 
 

Basics

Since the technique employs low energy electrons, it is necessarily restricted to use in high vacuum (HV) and UHV environments - however, the use of such low energy electrons ensures that it is a surface specific technique and, arguably, it is the vibrational technique of choice for the study of most adsorbates on single crystal substrates.

The basic experimental geometry is fairly simple as illustrated schematically below - it involves using an electron monochromator to give a well-defined beam of electrons of a fixed incident energy, and then analysing the scattered electrons using an appropriate electron energy analyser.

A substantial number of electrons are elastically scattered ( E = Eo ) - this gives rise to a strong elastic peak in the spectrum.

On the low kinetic energy side of this main peak ( E < Eo ), additional weak peaks are superimposed on a mildly sloping background. These peaks correspond to electrons which have undergone discrete energy losses during the scattering from the surface.

The magnitude of the energy loss, DE = (Eo - E), is equal to the vibrational quantum (i.e. the energy) of the vibrational mode of the adsorbate excited in the inelastic scattering process. In practice, the incident energy ( Eo ) is usually in the range 5-10 eV (although occasionally up to 200 eV) and the data is normally plotted against the energy loss (frequently measured in meV).

Selection Rules

The selection rules that determine whether a vibrational band may be observed depend upon the nature of the substrate and also the experimental geometry: specifically the angles of the incident and (analysed) scattered beams with respect to the surface.

For metallic substrates and a specular geometry, scattering is principally by a long-range dipole mechanism.  In this case the loss features are relatively intense, but only those vibrations giving rise to a dipole change normal to the surface can be observed.

By contrast, in an off-specular geometry, electrons lose energy to surface species by a short-range impact scattering mechanism.  In this case the loss features are relatively weak but all vibrations are allowed and may be observed.

If spectra can be recorded in both specular and off-specular modes the selection rules for metallic substrates can be put to good use - helping the investigator to obtain more definitive identification of the nature and geometry of the adsorbate species.

The resolution of the technique (despite the HREELS acronym !) is generally rather poor ; 40-80 cm-1 is not untypical. A measure of the instrumental resolution is given by looking at the FWHM (full-width at half maximum) of the elastic peak.

This poor resolution can cause problems in distinguishing between closely similar surface species - however, recent improvements in instrumentation have opened up the possibility of much better spectral resolution ( < 10 cm-1 ) and will undoubtedly enhance the utility of the technique.

In summary, there are both advantages and disadvantages in utilising EELS, as opposed to IR techniques, for the study of surface species It offers the advantages of ...

  • high sensitivity
  • variable selection rules
  • spectral acquisition to below 400 cm-1

but suffers from the limitations of ...

  • use of low energy electrons (requiring a HV environment and hence the need for low temperatures to study weakly-bound species, and also the use of magnetic shielding to reduce the magnetic field in the region of the sample)
  • requirement for flat, preferably conducting, substrates
  • lower resolution

Applications

One of the classic examples of an area in which vibrational spectroscopy has contributed significantly to the understanding of the surface chemistry of an adsorbate is that of molecular adsorption of CO on metallic surfaces. Adsorbed carbon monoxide usually gives rise to strong absorptions in both the IR and EELS spectra at the (C-O) stretching frequency. The metal-carbon stretching mode (ca. 400 cm-1 ) is usually also accessible to EELS. For a more detailed discussions on the bonding of CO to metals, you are recommended to refer to one of the following :
" Advanced Inorganic Chemistry " by F.A. Cotton & G. Wilkinson (5th Edn.) pp. 58 - 62. "Solids & Surfaces : a chemist's view of bonding in extended structures " by R. Hoffman pp. 71-74.

For more details and applications of vibrational specroscopies visit the website http://www.chem.qmul.ac.uk/surfaces/scc/scat5_4.htm

 

Instrumentation: Multifunctional System with Delta 0.5 Analyzer (SPECS)

Location: CME-124