High-resolution electron energy loss surface structure

Fig. VIII-10. (a) Intensity versus energy of scattered electron (inset shows LEED pattern) for a Rh(lll) surface covered with a monolayer of ethylidyne (CCH3), the structure of chemisorbed ethylene, (b) Auger electron spectrum, (c) High-resolution electron energy loss spectrum. [Reprinted with permission from G. A. Somoijai and B. E. Bent, Prog. Colloid Polym. ScL, 70, 38 (1985) (Ref. 6). Copyright 1985, Pergamon Press.]...

Figure 8.14 High-resolution electron energy loss spectroscopy (HREELS) and low-energy electron diffraction of CO adsorbed on a Rh(l 11) surface, along with structure models. The HREELS spectra show the C-O and metal-CO stretch vibrations of linear and threefold CO on rhodium (from R.Linke etal. [56]).

Recent studies using high resolution electron energy loss and photoelectron spectroscopy to investigate the effect of sulfur on the CO/Ni(100) system are consistent with an extended effect by the impurity on the adsorption and bonding of CO. Sulfur levels of a few percent of the surface nickel atom concentration were found sufficient to significantly alter the surface electronic structure as well as the CO bond strength. 

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. 

A few years ago, High Resolution Electron Energy Loss Spectroscopy (HREELS) - also named electron induced vibrational spectroscopy - has been successfully applied to characterize the composition and geometrical structure of polymer surfaces. In this review, the attributes of HREELS will be demonstrated and compared to the ones of other surface-sensitive spectroscopies. 

High Resolution Electron Energy Loss Spectroscopy has been "discovered" at about the same time as the previous cited techniques - die first reported experiment is related to a study of small molecules adsorbed on a (100)W surface and is dated from 1967 (1). During the last 15 years, the characterization of adsorption states of molecules on metal and semiconductor surfaces was the principal attribute of HREELS information on the elemental composition, on the chemistry, and the kinetics of surface reactions (versus temperature and/or time) were studied. One significant "plus" of HREELS is its ability to identify adsorption sites on a metal, by using the "dipole-selection rule" it is therefore possible to gain information on the short-scale structure or morphology of a surface with HREELS. 

G.D. Waddill and L.L. Kesmodel. Benzene Chemisorption on Palladium Surfaces. I. High Resolution Electron Energy Loss Vibrational Spectra and Structural Models. Phys. Rev. 5 31 4940 (1985). 

B.E. Koel, J.E. Crowell, C.M. Mate, and G.A. Somoijai. A High Resolution Electron Energy Loss Spectroscopy Study of the Surface Structure of Benzene Adsorbed on the Rhodium (111) Crystal Face. J. Chem. Phys. 88 1988 (1984). 

G.A. Somoijai, J.E. Crowell, R.J. Koestner, L.H. Dubois, and M.A. Van Hove. The Study of the Structure of Adsorbed Molecules on Solid Surfaces by High Resolution Electron Energy Loss Spectroscopy and Low Energy Electron Diffraction. In K. Fuwa, editor. Recent Advances in Analytical Spectroscopy. Pergamon Press, Oxford, 1982. 

The irreversible adsorption layer of aromatic species was investigated by means of a specially constructed UHV and electrochemistry system where surface structure is observed by low-energy electron diffraction (LEED), surface elemental composition and cleanliness are monitored by Auger spectroscopy (AES). The vibrational bonds of the adsorbed species is observed by high resolution electron energy loss spectroscopy (EELS). 

Figure 4. The surface structures of ethylene at 77K, 310K and 450K as determined by high resolution electron energy loss spectroscopy and low energy electron diffraction.

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