Electron loss spectroscopy, surface

Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Low-energy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate (7). Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately 1013 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures (< 10-4 Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. 

Fig. 1. Experimental techniques available for surface studies. SEM = Scanning electron microscopy (all modes) AES = Auger electron spectroscopy LEED = low energy electron diffraction RHEED = reflection high energy electron diffraction ESD = electron stimulated desorption X(U)PS = X-ray (UV) photoelectron spectroscopy ELS = electron loss spectroscopy RBS = Rutherford back scattering LEIS = low energy ion scattering SIMS = secondary ion mass spectrometry INS = ion neutralization spectroscopy.

Employing TERS in UHV systems There are a number of surface science tools available for samples in UHV which allow us to characterize the state of a surface. Surface and adlayer structures can be determined by LEED (low electron energy diffraction) as weU as by SPM (scanning probe microscopy) techniques. While the kind of chemical interactions can be studied, for example, with AES (Auger electron spectroscopy), EELS (energy electron loss spectroscopy) permits the identification of the chemical nature of the adsorbed species. TERS, on the other hand, may provide similar but also complementary information on the chemical identity under UHV conditions. As an additional advantage, TERS and SPM permit the identification and characterization of the spatial region from which this information is accumulated. 

The spectra consist of a series of sharp lines of the excited vibrational modes of the adsorbed molecules superimposed on a broad, enhanced background. Ethylene has been used to study the formation of intermediates on catalytic surfaces. Ethylene is chemisorbed dissociatively as acetylene at room temperature. This is revealed by the appearance of the C=C stretching vibration at 1204 cm and was confirmed by inelastic electron loss spectroscopy applied to acetylene chemisorbed on Ni(lll) surfaces. The strongest line in the spectmm of benzene chemisorbed at room temperature is the totally symmetric ring-breathing mode at 990 cm . All molecules with ring systems exhibit this characteristic band, it is the most strongly enhanced mode. 

Several techniques that provide information about composition and structure on the molecular level were discussed. For instance, secondary ion mass spectroscopy (SIMS), XPS which provide information about surface composition and the chemical environment and bonding of surface species, and ultraviolet photoelectron spectroscopy (UPS), which probes the density of electronic states in the valence band of materials. Also, the low energy electron diffraction (LEED) and high resolution energy electron loss spectroscopy (HREELS) are electronscattering techniques that are uniquely suited to yield the structure of the surface... 

Surface Characterization Using Spectroscopic Techniques. The elemental composition and the oxidation states of surfaces are most frequently determined by x-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS), Auger spectroscopy, and high-resolution electron loss spectroscopy (HREELS). 

Chemisorption and subsequent decomposition of bromomethane on a Mg(OOOl) single crystal surface under ultra high vacuum conditions were studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), temperature-programmed decomposition (TPD) and high-resolu-tion electron loss spectroscopy (EELS). 

Sexton BA (1979) Observation of formate species on a copper(lOO) surface by high resolution electron energy loss spectroscopy. Surface Sci 88 319-330 Sexton BA, Madix RJ (1981) A vibrational study of formic acid interaction with clean and oxygen-covered silver(llO) surfaces. Surface Sci 105 177-195 Sheldon RA, Kochi JK (1968) Photochemical and thermal reduction of cerium(IV) carboxylates. Formation and oxidation of alkyl radicals. J Am Chem Soc 90 6687-6698... 

The investigation of aqueous electrolyte-air interface encounters a couple of intrinsic challenges. Firstly, the majority of material is dissolved in the bulk and the interfacial region comprises only a tiny fraction of the total material of the system. Consequently, spectroscopic investigations with classical techniques such as Infrared, Raman or UV-spectroscopy are often hampered by the lack of surface specificity and the signals are dominated by bulk contributions. Secondly, the processes at the air-water interface are highly dynamic. On a molecular scale there is a tremendous traffic towards both adjacent bulk phases. Molecules evaporate and condense at the interface and diffuse towards the bulk phase. There is no defined static molecular arrangement and as a consequence, fairly broad spectral features are expected. Moreover, many powerful surface specific techniques such as electron loss spectroscopy have special requirements to the sample and the environment (e.g. UHV-conditions) and cannot be applied to the liquid-air interface. 

H. Ibach and D. L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic, New York, 1982. 

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. 

Figure Bl.25.12. Excitation mechanisms in electron energy loss spectroscopy for a simple adsorbate system Dipole scattering excites only the vibration perpendicular to the surface (v ) in which a dipole moment nonnal to the surface changes the electron wave is reflected by the surface into the specular direction. Impact scattering excites also the bending mode v- in which the atom moves parallel to the surface electrons are scattered over a wide range of angles. The EELS spectra show the higlily intense elastic peak and the relatively weak loss peaks. Off-specular loss peaks are in general one to two orders of magnitude weaker than specular loss peaks.

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. 

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