Theory of Dipole Electron Scattering from Adsorbates
Electron energy loss spectroscopy (EELS) is a relatively new tool for looking at adsorbate vibrations, but it is one which is rapidly expanding in popularity. The probe in this approach is a monoenergetic electron beam of energy E, where E is a few electron volts only. The beam is projected at the adsorbate covered surface at angle of incidence a, and the reflected beam is collected by an energy analyser whose resolution might be 2–10 meV, (see Fig. 1). The analyser may be fixed to look at the specular beam (early apparatus) or movable (later apparatus). In Fig. 2 is shown a typical spectrum, obtained for CO on Cu(100) at specular collection angle. One sees a very strong un-scattered peak at the original beam energy of 1.3 eV, and two loss peaks at 43 meV and 260 meV below the beam energy. These losses are due to excitation of adsorbate vibrations at these energies, the excitation appearing as an energy loss of the outgoing electron. The 260 meV loss is interpreted as a C-O stretch mode, and the 43 meV loss as a mode where the CO centre of mass vibrates against the surface (loosely called metal-carbon stretch).
KeywordsHydrocarbon Carbonyl Hunt Azimuth Deconvolution
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