Specular direction

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.

The lack of a well-defined specular direction for polycrystalline metal samples decreases the signal levels by 10 —10, and restricts the symmetry information on adsorbates, but many studies using these substrates have proven useful for identifying adsorbates. Charging, beam broadening, and the high probability for excitation of phonon modes of the substrate relative to modes of the adsorbate make it more difficult to carry out adsorption studies on nonmetallic materials. But, this has been done previously for a number of metal oxides and compounds, and also semicon-... 

Clearly, several spectra that exhibit strong type A features, e.g., those of ethyne on Ni(lll) or Pd(100), would merit reinvestigation, with careful attention being paid both to the identification of the specular direction and to the direction of the plane of incidence of the electron beam. If such experiments fail to account for type A/type A variations, the additional-species hypothesis could be tested by remeasuring systems giving type A spectra at markedly lower temperatures, when dissociative adsorption or self-hydrogenation would not be expected, and perhaps by deliberate postadsorption of ethene and/or perdeuterioethene. 

The HREELS, Auger electron spectroscopy (AES) and thermal desorption spectrometry (TDS) experiments were carried out in a UHV chamber described previously.6 Briefly, the chamber was equipped with a HREELS spectrometer for vibrational analysis, a single-pass cylindrical mirror analyzer for AES measurements and a quadrupole mass spectrometer for TDS measurements. The HREELS spectra were collected in the specular direction with an incident energy of 3.5 eV and with a spectroscopic resolution of 50-80 cm-1. The TDS data were obtained by simultaneously monitoring up to 16 masses, with a typical heating rate of about 1.5 K s-1. 

Features in VEEL spectra derived from the impact mechanism became more prominent in off-specular directions where the dipole-induced contributions are radically attenuated by the operation of the MSSR. However, impact-excited intensity can contribute to on-specular spectra in the form of additional strengths... 

A more detailed discussion of these mode assignments including reference to the IR spectra of model organometallic compounds is presented in reference 24. We assume the adorbate is oriented with its carbon-carbon axis approximately parallel to the surface since only small, broad peaks (1300 - 1400 cm l) are seen in the C-C stretching region. Observation of such a mode in the specular direction is prohibited by the normal dipole selection rule (3,... 

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