HREELS dipole scattering

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

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. 

Localised phonon excitations are in principle best studied by neutral-atom scattering, or off specular HREELS, in order to reduce the strong dipole excitation of Fuchs-Kliewer modes. Two off specular HREELS measurements on MgO(lOO) have been reported [25, 68], however there is some disagreement concerning the energy and assignment of the substrate derived loss peaks. Since the microscopic surface modes are expected to be sensitive to the surface structure, it has been suggested [9] that the differences may be associated with differences in surface preparation. 

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. 

The HCN ice spectrum illustrates several aspects of HREELS. The strong dynamic dipole moment due to the libration mode causes multiple scattering from the molecules, so that there are several replica primary peaks almost as intense as the elastic peak. Consequently, each of the usual HCN loss peaks generates a spectrum with the same spectral intensity relationships as the no-loss peak. The multiple... 

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