Energy electron loss spectroscopy molecules

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. 

Vibrational spectroscopy provides the most definitive means of identifying the surface species arising from molecular adsorption and the species generated by surface reaction, and the two techniques that are routinely used for vibrational studies of molecules on surfaces are Infrared (IR) Spectroscopy and Electron Energy Loss Spectroscopy (HREELS) (q.v.). 

Vibrations in molecules or in solid lattices are excited by the absorption of photons (infrared spectroscopy), or by the scattering of photons (Raman spectroscopy), electrons (electron energy loss spectroscopy) or neutrons (inelastic neutron scattering). If the vibration is excited by the interaction of the bond with a wave... 

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 ... 

In order to elucidate the results of the CO TPD experiment, the detailed structure of the oxygen-modified Mo(l 12) surfaces and the adsorption sites of CO on these surfaces have been considered. Zaera et al. (14) investigated the CO adsorption on the Mo(l 10) surface by high-resolution electron-energy-loss spectroscopy (HREELS) and found vatop sites. Francy et al. (75) also found a 2100 cm loss for CO on W(IOO) and assigned it to atop CO. Recently, He et al. (16) indicated by infrared reflection-absorption spectroscopy that at low exposures CO is likely bound to the substrate with the C-0 axis tilted with respect to the surface normal. They, however, have also shown that CO molecules adsorbed on O-modified Mo(l 10) exhibi Vc-o 2062 and 1983 cm L characteristic to CO adsorbed on atop sites. Thus it is supposed that CO adsorbs on top of the first layer Mo atoms. 

Welipitiya etal. [132] have studied adsorption and desorption of ferrocene on Ag(lOO), applying photoemission and thermal desorption. The initially adsorbed surface species closely resembled that of molecular ferrocene. The molecule was adsorbed with the cyclopentadienyl ring ligands parallel to the surface. Wood-bridge etal. [133] have performed the high-resolution electron energy loss spectroscopy (HREELS) andXPS studies of ferrocene on Ag(lOO). Researchers from the... 

A versatile tool to analyze vibrations of surface atoms and adsorbed molecules is high-resolution electron energy loss spectroscopy (HREELS) [359], Monoenergetic low-energy electrons (1-10 eV) are directed to the surface. Most of them are backscattered elastically. 

The final chapters of this book review the progress of three recently developed techniques that provide information about the vibrational states of adsorbed molecules. Perhaps the most important of these techniques is electron energy loss spectroscopy that, despite its inherent low resolution, gives valuable information on vibrational modes that are either inactive in the IR, or inaccessible because of experimental difficulties. The applications of this technique are discussed in two chapters by Somorjai and Weinberg. The review of new experimental techniques concludes with presentations on inelastic electron tunneling spectroscopy and neutron scattering by Kirtley and Taub. 

The Application of High Resolution Electron Energy Loss Spectroscopy to the Characterization of Adsorbed Molecules on Rhodium Single Crystal Surfaces... 

Ni(001) where dramatic differences in the molecule yield are predicted to occur with molecular orientation.(14) (iii) Electron energy loss spectroscopy indicates that pyridine on Ag(lll) initially adsorbs in ir-bonded configuration but undergoes a compressional phase transition to a bonded configuration as the coverage is increased. (15) Benzene, on the other hand, is believed to remain in the ir-bonded configuration at all coverages. (16) A more detailed discussion of these effects is presented in reference 1. 

The preference of Eq. (21a) for C02 dissociation may be well anticipated. It has been shown [see, for example, the high-resolution electron energy loss spectroscopy (HREELS) studies of C02 on Re(001) (71a), ultraviolet/ X-ray photoelectron spectroscopy (UPS/XPS) studies of C02 on Fe(lll) and Fe(l 10) (37), and computer simulations for C02 on Pt(l 11) (71b)] that the molecule is practically undistorted (symmetric and linear) in the ground chemisorbed state but strongly distorted (nonsymmetric and bent) as an intermediate preceding the dissociation C02 s — COs + Os. So, there is a good reason to believe that in the transition state the coordinated C—O bond is strongly expanded [by 0.12 A (71b)] and becomes very weak. But the weaker the C—O bond (xc0 — 0), the more accurate is Eq. (21a). 

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