Elastic peak

Figure Al.7.12. Secondary electron kinetic energy distribution, obtained by measuring the scadered electrons produced by bombardment of Al(lOO) with a 170 eV electron beam. The spectrum shows the elastic peak, loss features due to the excitation of plasmons, a signal due to the emission of Al LMM Auger electrons and the inelastic tail. The exact position of the cutoff at 0 eV depends on die surface work fimction.

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.

L of CO was adsorbed at a pressure of 1 x 10 mbar and T= 200 K. At zero energy loss one observes the highly intense elastic peak. The other peaks in the spectrum are loss peaks. At high energy, loss peaks due to dipole scattering are visible. In this case they are caused by CO vibration perpendicular to the surface. The... 

The Q and ft) dependence of neutron scattering structure factors contains infonnation on the geometry, amplitudes, and time scales of all the motions in which the scatterers participate that are resolved by the instrument. Motions that are slow relative to the time scale of the measurement give rise to a 8-function elastic peak at ft) = 0, whereas diffusive motions lead to quasielastic broadening of the central peak and vibrational motions attenuate the intensity of the spectrum. It is useful to express the structure factors in a form that permits the contributions from vibrational and diffusive motions to be isolated. Assuming that vibrational and diffusive motions are decoupled, we can write the measured structure factor as... 

FIGURE 27.45 HREELS spectra for Pd(lll) after emersion from lOOmM CFjCOOH + 1 mM benzene electrolyte at (a) 0.5 V and (b) 0.2 V RHE. The peak intensities were normalized with respect to the elastic peak. (From Kim et ah, 2003, with permission from Elsevier.)... 

Figure 8. HREELS spectra of the nanocrystalline diamond and diamond-like carbon films with various [CH4]/[CO]. (a) [CH4]/[CO] = 4.5.0/0. (b) [CH4MCO] = 4.5/1.0. (c) [CH4]/[CO] = 4.5/10 seem. The elastic peak for (c), reduced by a factor of 25, is shown for comparison. Reprinted with permission from [66], K. Okada et al.. Diamond Relat. Mater. 10, 1991 (2001). 2001, Elsevier Science.

Nuclear absorption of incident X-rays (from the synchrotron beam) occurs elastically, provided their energy, y, coincides precisely with the energy of the nuclear transition, Eq, of the Mossbauer isotope (elastic or zero-phonon peak at = E m Fig. 9.34). Nuclear absorption may also proceed inelasticaUy, by creation or annihilation of a phonon. This process causes inelastic sidebands in the energy spectrum around the central elastic peak (Fig. 9.34) and is termed nuclear inelastic scattering (NIS). 

A typical loss spectrum is recorded over a range of about 1000 eV, and is conventionally considered to consist of three regions. The first zero-loss, or elastic peak represents electrons which are transmitted without suffering any measurable energy loss. The low loss region, containing electrons which have lost up to about... 

A substantial number of electrons are elastically scattered, and this gives rise to a strong elastic peak in the spectrum. When an electron of low energy (2-5 eY) approaches a surface, it can be scattered inelastically by two basic mechanisms, and the data obtained are dependent upon the experimental geometry - specifically the angles of the incident and the (analysed) scattered beams with respect to the surface (0 and 02 in Figure 5.47). Within a certain distance of the surface the incident electron can interact with the dipole field associated a particular surface vibration, e.g. either the vibrations of the surface atoms of the substrate itself, or one or other... 

Figure 8.14 shows HREELS and LEED measurements of CO adsorbed on Rh(l 11) (56]. The bottom spectrum is that of the empty surface and shows only the elastic peak, which has a full width at half maximum of about 2 meV, or 16 cm 1. In fact, such a spectrum provides a stringent test for the cleanliness of the surface, as any impurity, e.g. of adsorbed carbon or oxygen, would immediately result in a peak in the 300-600 cm-1 region of the HREELS spectrum. The LEED image shows the... 

The intensity of the HREELS peaks, including that of the elastic peak, depends strongly on the adsorbate and the degree of ordering on the surface. Therefore it is common practice to scale all spectra with respect to the elastic peak, as has been done for the spectra in Fig. 8.14. 

Besides the inelastic component, always a certain number of He atoms are elastically scattered in directions lying between the coherent diffraction peaks. We will refer to this scattering as diffuse elastic scattering. This diffuse intensity is attributed to scattering from defects and impurities. Accordingly, it provides information on the degree and nature of surface disorder. It can be used for example to study the growth of thin films or to deduce information on the size, nature and orientation of surface defects Very recently from the analysis of the diffuse elastic peak width, information on the diffusive motion of surface atoms has been obtained. ... 

Figure 4.26. B and Q band intensities, normalized to the elastic peak intensity as a function of CuPc coverage. Reprinted with permission from A. Ruocco, M. P. Donzello, F. Evangelista and G. Stefani, Physical Review B, 67, 155408 (2003). Copyright (2003) by the American Physical Society.

Fig. 30. Schematic representation of the backscattered secondary-electron spectrum associated with an (isolated) external source of monoenergetic electrons of energy E. "ntis logj(E) vs. log (E) display mode emphasizes the separation of the spectrum into three parts (1) the secondary cascade which is bounded at high energies (at E p) by (2) the region of rediffused primaries which is bounded by (3) the elastic peak. At low energies the cascade is attenuated by the escape probability P(E). Auger and characteristic loss processes, among other things are not included in this idealized spectrum. (From Ref. " )...

Fig. 4.8. High-resolution electron energy loss speetra for H and D adsorbed atomically on W(IOO). The elastic peak is shown at left. The loss energy for hydrogen is plotted along the horizontal axis. The coverage varies from 0 = 0.4 to 6 = 2.0 (saturation), exhibiting a change in adsorption site, while the deuterium spectrum is shown at 0 = 2.0 only. [After H. Froitzheim, H. Ibach and S. Lehwald, Phys. Rev. Lett. 36, 1549 (1976).]...

The two situations of common ocurrence are for a bulk homogeneous material for which the intensity of the elastic peak is given as in Eq. (1), and for an overlayer of thickness d for which expressions (2) and (3) are obtained. 

For bulk homogeneous material A of thickness essentially infinite compared with the typical electron mean free paths the intensity of the elastic peak. Ia is given by Eq. (1). 

The Rh(lll) surface was covered with carbon by decomposing 5 x 10 7 torr of either acetylene or ethylene at 1100 K for 10 minutes and subsequent flashing to 1200 K (230. Pre-adsorbed carbon had a very strong inhibiting effect on carbon monoxide chemisorption. This is the same effect it had on the methanation rate (36). The low inelastic scattering intensity indicated relatively small CO coverages while the broad elastic peak and... 

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