In this section, we mainly deal with the interpretation of spectra of HREELS, and find what we learn from it.
The interpretation of HREELS spectra follows that of RAIRS. Bear in mind that RAIRS spectra are cast usually cast in wavenumbers while HREELS are in meV (1 meV = 8.065 cm-l). The resolution in RAIRS is typically 0.25 meV while it is about 20 meV for HREELS. By comparison with the RAIRS , a spectrum of the same material taken both by RAIRS and HREELS is shown in Fig. Note the additional spectra given here, demonstrating the molecular information obtainable.
Primarily one gains insight to the real molecular nature of adsorbates, that is, if some dissociation has occurred during adsorption, HREELS can help elucidate that.
(For example, hydrogen adsorbs readily onto a variety of transition metal surfaces. An issue of fundamental impor- tance is whether the adsorbed entity is an H2 molecule, or whether the molecule dissociates during the adsorption process to leave two hydrogen atoms, each of which bonds to the surface as a separate entity.Of course, one may envision the possibility of more than one bonding site, and the possibility of both the molecular and atomic form of hydrogen on the surface simultaneously. If hydrogen is adsorbed on the surface in molecular form, then the vibrational spectrum of the surface should show a feature near the gas phase H2 vibrational frequency of 4560 cm- 1 (550 meV). This will be the case so long as the hydrogen bond is not dramatically altered by the adsorption process. When the vibrational spectrum of hydrogen on transition metal surfaces is obtained, one obtains features in the range 800-1600cm-1, well above the substrate vibrational frequencies but far below that of the very high frequency H2 stretching mode. Thus, from this alone it is clear that the hydrogen adsorbs on the surface in atomic form, rather than as a molecular entity)
Furthermore, the vibrational frequency of, for instance, CO in an on-top site vs. a three-fold hollow site can be expected to be different.
Also, different modes might be expected for different binding sites.
Hence HREELS is extremely important for increasing our understanding about the details of adsorption. Much can be learned of the structure of the surface, and of entities adsorbed on it, through knowledge of the characteristic vibrational frequencies.
HREELS still can probe surface phonon. An example is shown in Fig.
Example of a feature in a HREELS spectrum due to surface phonon scattering, from Ibach et al. The peak at 56meV from the Si(111)-(2x1) surface prepared by clevage in vacuo, at a primary energy of 5eV, has been ascribed to such scattering. Note the rapid damping of such a peak on exposure
to oxygen; The half-width of the loss feature is the same as that of the elastic beam, which conforms the surface origin.