HREELS impact scattering

Figure 12 In-specular (bottom spectrum) and off-specular (middle and top) HREELS measurements for 0/Ag(21 0), recorded for the same electron energy and for the same scattering angle, 0S. The loss at 56meV has a remarkably strong impact scattering component which leads to an inversion of the intensity ration with the 40 meV loss for out-of-specular conditions.

A third mechanism, first observed in gas-phase electron-impact scattering, has been referred to as negative-ion resonance. In this process, an electron is trapped, within 10 s, inside the molecule in a negative-ion state. For chemisorbed molecules, however, the adsorbate-substrate chemical bond and the electron-surface interactions can dramatically alter the resonance properties. Hence, for HREELS at metal surfaces, this mechanism is quite rare it will not be treated further in this article. 

HREELS (Table 4.1) has the advantage of detecting all types of vibration this is because there are two excitation mechanisms, viz. dipole scattering and impact scattering. The former is subject to the same selection rules as RAIRS and gives strong features on-specular, but the latter excites all vibrational modes. There are however supplementary selection rules that apply to impact scattering in the on-specular direction. As noted earlier, this technique is not applicable to supported metal catalysts. 

There are three important scattering mechanisms in HREELS dipole scattering, impact scattering and negative ion resonance scattering. 

Since the parallel components of the dynamic dipole are active in RAIRS, it is possible to use the azimuthal dependence to obtain the orientation of the adsorbate at the surface. A similar technique has been applied to adsorbates on metals in HREELS measurements made off specular in order to observe parallel modes through impact or resonant scattering processes. This was first demonstrated for the Rh(CO)2 molecule on anisotropic TiO2(110) surface [72]. The results of this study also allow a test of the three layer model theory (Fig.5,6) as applied to S-polarised radiation. Fig. 11 shows the FT-RAIRS spectrum for 1/3 monolayer of Rh(CO)2 on Ti02(l 10) measured with P and S polarised radiation. 

Fig. 2. Ewald sphere construction for the inelastic scattering events. For He atoms the energy loss is comparable to the impact energy of the particles, while for electrons it is negligible on the scale of the figure. The HREELS spectra are therefore effectively constant q, scans, while HATOF spectra run along the so-called scan curves and include losses with different q, values.

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