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).
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
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
System with Delta 0.5 Analyzer (SPECS)