HREELS

In order to deduce the orientation of the chemisorbed molecules, the metal-surface dipole selection mle needs to be invoked this states that only vibrations with components perpendicular to the metal surface are HREELS-active. That is, if benzene 

Such indications are in consonance with the EC-STM results in that  

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HREELS High-resolution electron energy-loss spectroscopy [129, 130] Same as EELS Identification of adsorbed species through their vibrational energy spectrum... 

Fig. XVni-4. HREELS vibrational spectra from ethylene chemisorbed on Rh(lll). There is a sequential dehydrogenation on heating. (From Ref. 13, p. 64.)...

Fig. XVin-5. HREELS spectra for NO2 on Pt(lll) adsorbed in different bonding geometries. [From M. E. Bartram and B. E. Koel, / Vac. Sci. Tech., A6, 782 (1988).]...

As the table shows, a host of other teclmiques have contributed a dozen or fewer results each. It is seen that diffraction teclmiques have been very prominent in the field the major diffraction methods have been LEED, PD, SEXAFS, XSW, XRD, while others have contributed less, such as NEXAFS, RHEED, low-energy position diffraction (LEPD), high-resolution electron energy loss spectroscopy (HREELS), medium-energy electron diffraction (MEED), Auger electron diffraction (AED), SEELFS, TED and atom diffraction (AD). 

Analysis of Surface Molecular Composition. Information about the molecular composition of the surface or interface may also be of interest. A variety of methods for elucidating the nature of the molecules that exist on a surface or within an interface exist. Techniques based on vibrational spectroscopy of molecules are the most common and include the electron-based method of high resolution electron energy loss spectroscopy (hreels), and the optical methods of ftir and Raman spectroscopy. These tools are tremendously powerful methods of analysis because not only does a molecule possess vibrational modes which are signatures of that molecule, but the energies of molecular vibrations are extremely sensitive to the chemical environment in which a molecule is found. Thus, these methods direcdy provide information about the chemistry of the surface or interface through the vibrations of molecules contained on the surface or within the interface. 

High-Resolution Electron Energy Loss Spectroscopy (HREELS) 1.8.3... 

Sc yrodcT. Ninth Conference on Electron Microscopy. 1978, vol. l,p.534. (Microscopial Society of Canada, Toronto, Canada) Unique combination of Eg = 30 keV used in HREELS study of -Si (H) films with meV energy resolution. 

In this chapter, three methods for measuring the frequencies of the vibrations of chemical bonds between atoms in solids are discussed. Two of them, Fourier Transform Infrared Spectroscopy, FTIR, and Raman Spectroscopy, use infrared (IR) radiation as the probe. The third, High-Resolution Electron Enetgy-Loss Spectroscopy, HREELS, uses electron impact. The fourth technique. Nuclear Magnetic Resonance, NMR, is physically unrelated to the other three, involving transitions between different spin states of the atomic nucleus instead of bond vibrational states, but is included here because it provides somewhat similar information on the local bonding arrangement around an atom. 

Fundamental information from vibrational spectra is important for understanding a wide range of chemical and physical properties of surfaces, e.g., chemical reactivity and forces involved in the atomic rearrangement (relaxation and reconstruction) of solid surfaces. Practical applications of HREELS include studies of ... 

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

The intensity of a vibrational mode in HREELS on-specular is given by the ratio of the inelastic to elastic intensities... 

The width and shape of the energy loss peaks in HREELS are usually completely determined by the relatively poor instrumental resolution. This means that no information can be obtained from HREELS about such interesting chemical physics questions as vibrational energy transfer, since the influence of the time scale and mechanism of vibrational excitations at surfaces on the lifetimes, and therefore the line widths and shapes, is swamped. (Adsorbates on surfaces have intrinsic vibra-... 

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