Impact scattering

The second excitation mechanism, impact scattering, involves a short range interaction between the electron and the molecule (put simply, a collision) which scatters the electrons over a wide range of angles. The usefiil feature of impact scattering is that all vibrations may be excited and not only the dipole active ones. As in Raman spectroscopy, the electron may also take an amount of energy hv away from excited molecules and leave the surface with an energy equal to Eq + hv. 

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.

Where infrared and Raman spectroscopy are limited to vibrations in which a dipole moment or the molecular polarizability changes, EELS detects all vibrations. Two excitation mechanisms play a role in EELS dipole and impact scattering. 

The energy analysis of these inelastically scattered electrons is carried out by a cylindrical sector identical to the monochromator. The electrons are finally detected by a channeltron electron multiplier and the signal is amplified, counted and recorded outside of the vacuum chamber. A typical specularly reflected beam has an intensity of 10 to 10 electrons per second in the elastic channel and a full width at half maximum between 7 and 10 meV (60-80 cm l 1 meV = 8.065 cm-- -). Scattering into inelastic channels is between 10 and 1000 electrons per second. In our case the spectrometer is rotatable so that possible angular effects can also be studied. This becomes important for the study of vibrational excitation by short range "impact" scattering (8, 9, 10). 

The loss intensities are a strong function of the incident electron energy and only average values are reported here. This could be due to molecular resonances or short range "impact" scattering ( , 9, 1(), 96) as discussed by Lehwald and Ibach for the case of acetylene chemisorbed or Ni(lll)... 

The impact scattering or sudden mechanism has been used to explain the vibrational excitation of chemisorbed molecules in electron energy loss spectra (EELS). See in particular the reference by Tong et al. [25]. 

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.

The EELS spectrum of DMBM [Fig. 39(a)] and the IR spectrum of solid DMBM in KBR [Fig. 39(b)] are very similar except for the absence of the sulfhydryl SH stretch in the EELS spectrum (2500 cm-1). [The difference in appearance of the EELS and IR spectra results from several causes the resolution of EELS (80 cm-1) was poorer than that of IR (4 cm-1) EELS emphasizes electron impact scattering while IR emphasizes dipole scattering, an experimentally valuable distinction and the intrinsic line widths may be broader for the adsorbed layer than for the solid compound.] Assignments of the EELS bands are proposed in ref. 81. Absence of the SH stretch from EELS spectra of DMBM is evidence for removal of the sulfhydryl hydrogen during adsorption... 

In the case of impact scattering, an atomistic description is essential, since a short-range interaction is operative. The cross-section is then (Eqs. 3.77-79 of Ibach and Mills/61/)... 

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