Surface vibration inelastic scattering

In electron energy loss spectroscopy an electron incident on a surface is inelastically scattered and the distribution of electrons n(AE) having transferred an energy AE to the target is measured. Both vibrational and electronic transitions can be excited, but this section will be confined to ELS in the range of electronic excitations. 

SANS Small-angle neutron scattering [175, 176] Thermal or cold neutrons are scattered elastically or inelastically Incident-Beam Spectroscopy Surface vibrational states, pore size distribution suspension structure... 

Electrons interact with solid surfaces by elastic and inelastic scattering, and these interactions are employed in electron spectroscopy. For example, electrons that elastically scatter will diffract from a single-crystal lattice. The diffraction pattern can be used as a means of stnictural detenuination, as in FEED. Electrons scatter inelastically by inducing electronic and vibrational excitations in the surface region. These losses fonu the basis of electron energy loss spectroscopy (EELS). An incident electron can also knock out an iimer-shell, or core, electron from an atom in the solid that will, in turn, initiate an Auger process. Electrons can also be used to induce stimulated desorption, as described in section Al.7.5.6. 

Perhaps the best known and most used optical spectroscopy which relies on the use of lasers is Raman spectroscopy. Because Raman spectroscopy is based on the inelastic scattering of photons, the signals are usually weak, and are often masked by fluorescence and/or Rayleigh scattering processes. The interest in usmg Raman for the vibrational characterization of surfaces arises from the fact that the teclmique can be used in situ under non-vacuum enviromnents, and also because it follows selection rules that complement those of IR spectroscopy. 

The other technique is HREELS (high resolution EELS) which utilises the inelastic scattering of low energy electrons in order to measure vibrational spectra of surface species. The use of low energy electrons ensures that it is a surface specific technique, and is often chosen for the study of most adsorbates on single crystal substrates. 

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 3.17. Vibrational excitation of NO (v = l)/v = 0) in direct inelastic scattering from Ag(111), (a) as a function of the surface temperature Ts at two incident energies. The straight lines are Arrhenius fits with an effective activation energy of the NO vibrational frequency. The two sets of data at the different Et are arbitrarily scaled relative to each other for clarity, (b) as a function of incident normal energy En at Ts = 760 K. From Ref. [175].

Figure 6. Comparison of observed and calculated vibrational spectra for a butane monolayer (19). (Topi Observed spectrum for monolayer butane adsorbed on a graphitized carbon powder at 80 K. The background inelastic scattering from the substrate has been subtracted, (a) Calculated spectrum for the butane molecule adsorbed with its carbon skeleton parallel to the graphite layers and the bottom layer of four hydrogen atoms bonded to the surface with force constants listed in Table I. (b) Same orientation but only the carbon atoms are bonded to the surface with a force constant of 0.12 mdyn/A. (Bottom) Butane carbon plane perpendicular to the graphite layers and the bottom layer of four hydrogen atoms bonded to the surface with the same force constants as in the parallel orientation.

Inelastic scattering of neutrons yields neutron scattering spectra that measure the vibrational energy levels of the material under study. For example, the chemisorption of water on Raney nickel was shown by inelastic scattering both to produce hydroxyl groups and to chemisorb water molecules on the surface at less than monolayer coverages. 

Another class of techniques monitors surface vibration frequencies. High-resolution electron energy loss spectroscopy (HREELS) measures the inelastic scattering of low energy ( 5eV) electrons from surfaces. It is sensitive to the vibrational excitation of adsorbed atoms and molecules as well as surface phonons. This is particularly useful for chemisorption systems, allowing the identification of surface species. Application of normal mode analysis and selection rules can determine the point symmetry of the adsorption sites./24/ Infrarred reflectance-adsorption spectroscopy (IRRAS) is also used to study surface systems, although it is not intrinsically surface sensitive. IRRAS is less sensitive than HREELS but has much higher resolution. 

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