HREELS, scattering

Figure 4 A schematic representation of specular reflection (0) and momentum conservation in HREELS scattering events. The momentum parallel to the sur ce is conserved. The quantity Q is the parallel momentum transfer, and determines the angular deflection A0. The incident electron is characterized by velocity and momentum components vand ftp, ft is the scattered electron momentum, and Aftj the change in momentum in the z direction.

While the underlying mechanisms of HREELS are pretty well understood, many important details relating to selection rules and scattering cross sections remain unknown. 

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. 

As noted in the introduction, vibrations in molecules can be excited by interaction with waves and with particles. In electron energy loss spectroscopy (EELS, sometimes HREELS for high resolution EELS) a beam of monochromatic, low energy electrons falls on the surface, where it excites lattice vibrations of the substrate, molecular vibrations of adsorbed species and even electronic transitions. An energy spectrum of the scattered electrons reveals how much energy the electrons have lost to vibrations, according to the formula ... 

By scattering within molecular solids and at their surfaces, LEE can excite with considerable cross sections not only phonon modes of the lattice [35,36,83,84,87,90,98,99], but also individual vibrational levels of the molecular constituents [36,90,98-119] of the solid. These modes can be excited either by nonresonant or by resonant scattering prevailing at specific energies, but as will be seen, resonances can enhance this energy-loss process by orders of magnitude. We provide in the next two subsections specific examples of vibrational excitation induced by LEE in molecular solid films. The HREEL spectra of solid N2 illustrate well the enhancement of vibrational excitation due to a shape resonance. The other example with solid O2 and 02-doped Ar further shows the effect of the density of states on vibrational excitation. 

In order to be effective, EELS spectrometers must satisfy a number of stringent requirements. First, the primary electrons should be monochromatic, with as little spread in energy as possible, preferably around 1 meV or better (1 meV = 8 cm-1). Second, the energy of the scattered electrons should be measured with an accuracy of 1 meV, or better. Third, the low-energy electrons must effectively be shielded from magnetic fields. The resolution of EELS has steadily been improved, from typically 50 to 100 cm-1 around 1975 to better than 20 cm-1 for the currently available spectrometers. When the latter value comes close to the line width of a molecular vibration, the technique is usually referred to as high-resolution EELS (HREELS). 

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.

ELS Electron Energy Loss Spectroscopy Monoenergetic electrons 5-50 eV are scattered off a surface the energy losses are measured. This gives information on the electronic excitations of the surface and adsorbed molecules (see HREELS). Sometimes called EELS. 

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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