Sensitivity, HREELS

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. 

HREELS and TFD have played a unique role In characterizing the surface chemistry of systems which contain hydrogen since many surface techniques are not sensitive to hydrogen. We have used these techniques to characterize H2S adsorption and decomposition on the clean and (2x2)-S covered Ft(111) surface (5). Complete dissociation of H,S was observed on the clean Ft(lll) surface even at IlOK to yield a mixed overlayer of H, S, SH and H2S. Decomposition Is primarily limited by the availability of hydrogen adsorption sites on the surface. However on the (2x2)-S modified Ft(lll) surface no complete dissociation occurs at IlOK, Instead a monolayer of adsorbed SH Intermediate Is formed (5) ... 

While the detection of the Si-H and Si-C modes indicates HREELS can probe the buried molecule/silicon interface, in general this method will be most sensitive to the terminal groups at the vacuum/monolayer interface. This is illustrated in Fig. 9 where spectra for several modified surfaces with different terminal functionalities are shown. In each case this terminal group is tethered to the surface via a Cio alkyl linker yet the spectra are significantly different. This is particularly evident in the spectra for the thienyl terminated surface in which the aromatic C-H stretch is clearly observed. In contrast this mode is quite small in the FTIR spectra, which are dominated by the contributions of the alkyl linker chain [51]. The observation of strong terminal group modes in the HREELS spectra indicates that these functional groups are likely present at the surface of the film and not buried back towards the H-terminated surface. This is consistent with their availability for sequential reactions as discussed in the previous section. 

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. 

Sensitivity an intensity-resolution compromise regarding IR spectroscopy. It is probably mandatory first to evidence that the HREELS spectroscopy allows one to record a useful vibrational spectrum, i.e. to detect at least the molecular groups... 

To summarize this section, we present HREELS as a new spectroscopy, with attributes similar to the IR ones, but with a very high surface sensitivity, and a different quantification procedure. 

Another class of techniques monitors surface vibration frequencies. High-resolution electron energy loss spectroscopy (HREELS) measures the inelastic scattering of low energy ( 5eV) electrons from surfaces. It is sensitive to the vibrational excitation of adsorbed atoms and molecules as well as surface phonons. This is particularly useful for chemisorption systems, allowing the identification of surface species. Application of normal mode analysis and selection rules can determine the point symmetry of the adsorption sites./24/ Infrarred reflectance-adsorption spectroscopy (IRRAS) is also used to study surface systems, although it is not intrinsically surface sensitive. IRRAS is less sensitive than HREELS but has much higher resolution. 

Fundamental investigations of the interactions of (reacting) gas molecules with single crystal and nanoparticle model catalysts have largely been carried out under UHV, with a number of surface-sensitive techniques, such as LEED, TPD, HREELS, IRAS, AES, XPS, UPS, and others being applied (3,33,34). Unfortunately, these methods typically cannot be used under catalytic reaction conditions (> 1 bar), for example, because of mean free path restrictions of the involved electrons, atoms, or ions. 

HREELS generally does not possess enough sensitivity to detect higher-order effects due to mechanical and/or electrical anharmonicity. 

This is a surface vibrational spectroscopic technique that involves the irradiation of the adsorbate-metal interface with a beam of low-energy (2 to 10 eV) electrons and the measurement of the energies of the backscattered electrons energy losses below 0.5 eV are due mainly to inelastic interactions with metal-surface phonons and adsorbate vibrational excitations. The extremely high sensitivity of HREELS makes possible measurements of adsor-... 

At the time of a recent review [9], there remained very few examples of vibrational studies of adsorbate, or localised substrate modes, at metal oxide surfaces. By far the majority of studies concerned the characterisation by HREELS of phonon modes (such as Fuchs-Kliewer modes) pertaining to the properties of the bulk structure, rather than the surface, or to electronic transitions. Such studies have been excluded from this review in order to concentrate on the vibrational spectroscopy of surface vibrations on well-characterised metal oxide surfaces such as single crystals or epitaxially grown oxide films, for which there is now a substantial literature. Nevertheless, it is important to briefly describe the electronic and phonon properties of oxides in order to understand the constraints and difficulties in carrying out RAIRS and HREELS with sufficient sensitivity to observe adsorbate vibrations, and more localised substrate vibrational modes. 

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