Electron loss spectroscopy

High Resolution Electron Loss Spectroscopy (HREELS) 194... 

HIGH RESOLUTION ELECTRON LOSS SPECTROSCOPY (HREELS)... 

Electron loss spectroscopy (EELS) Composition Energy states of adsorbed species Electrons in, electrons out Electrons in, electrons out... 

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. 

Another interesting and different type of catalysis is involved in the catalyzed reconstruction of an indium oxide overlayer on indium. This study was alluded to earlier in the discussion of acetate ion species formed on indium oxide by chemisorption from several torr of acetic acid gas. At low partial pressures of acetic acid (<< 0.1 torr) the reversible adsorption of acetic acid catalyzes the reconstruction of a thin ( 10-15A), porous indium oxide overlayer to a defect-free (no pin holes) film as judged by pinhole sensitive tunnel junction measurements. Some clues as to the mechanism were obtained from IR plus Auger and electron loss spectroscopy as well as ellipsometry measurements. The overall process is shown in Fig. 8. This is an example where processes in the substrate themselves can be usefully catalyzed. 

Electron loss spectroscopy (ELS, HREELS) electrons/same electrons 0.5-2 electronic and vibronic excitation... 

HRELS high-resolution electron loss spectroscopy... 

The Hertz experiment may also be regarded as the forerunner of the recently revived technique (Kuppermann et al., 1962) of electron loss spectroscopy applied to vapours. In this, the characteristic energy losses (Vc) are revealed as groups of scattered electrons of energy volts less... 

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.

The X-ray diffraction patterns for Ge02 (H) and Ge02 (T) have been accurately determined and can be used to identify films as thin as 500 A (using low angle diffraction). Here again, the monoxide phase cannot be identified by this means since no diffraction pattern has been observed for that phase. Some other non-destructive techniques have been used such as low energy electron diffraction (LEED), electron loss spectroscopy (ELS), Raman scattering, etc. but usually they are so sensitive to contamination that the results cannot easily be used for simple phase identification. Such techniques are therefore more useful for physical property studies. 

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). 

One critical spectroscopic measurement of C50 was the determination of the gap between the HOMO to LUMO by electron loss spectroscopy in 1991. 

For dipole forbidden transitions, e.g., from a p core state to a 4f final state, higher-order terms in the exponential expansion lead to a finite contribution to the cross section. This is in general small unless q 1/r, where is the core orbital radius, but such conditions may be satisfied both in transmission and reflection geometry (Schnatterly 1979, Grunes and Leapman 1980, Ludeke and Koma 1975), and will be of importance in rare earth electron loss spectroscopy. Thus even within the Born-Bethe regime monopole, quadrupole and octopole transitions may be prominent. 

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