Applications of HREELS

One of the classic examples of an area in which vibrational spectroscopy has contributed to the understanding of the surface chemistry of an adsorbate is that of the molecular adsorption of CO on metallic surfaces. Adsorbed CO usually gives rise to strong absorptions in both the IR and HREELS spectra at the (C-O) stretching frequency. The metal-carbon stretching mode ( 400 cm-1) is usually also accessible to HREELS. 

The vibrational modes of molecules adsorbed on a surface provide one with direct information on the nature of the chemical bonds between a molecule and its 

Briggs, D. Seah, M.P. Eds. (1990) Practical Surface Analysis (Second Edition) Vol. 1, John Wiley Sons, Chichester. 

Egerton, R.F. (1986) Electron Energy-Loss Spectroscopy in the Electron Microscope, Plenum Press, New York. 

Flewitt, P.E.J. Wild, R.K. (1985) Microstructural Characterisation of Metals and Alloys, The Institute of Metals, London. 

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Fundamental information from vibrational spectra is important for understanding a wide range of chemical and physical properties of surfaces, e.g., chemical reactivity and forces involved in the atomic rearrangement (relaxation and reconstruction) of solid surfaces. Practical applications of HREELS include studies of ... 

My feeble attempts were initially made two decades ago in the application of HREELS to the study of polymer surfaces. The efforts did not become more earnest until a... 

In this section the application of HREELS to the study of polymeric surfaces will be reviewed with relevance for some of its specific aspects quantitative application of the vibrational studies and electronic studies. 

As the purpose of this section is to give the flavour of the potential applications of HREELS, no funher detail will be given. A review has recently been dedicated to the application of HREELS to metal-polymer imerfaces. ... 

The above description indicates that the applications of HREELS have been entirely basic and concerned with the interactions of molecules with clean or well-characterized surfaces. A very typi-... 

Lazar S, Botton GA, Wu M-Y, Tichelaar FD, Zandbergen HW. (2003) Materials science applications of HREELS in near edge structure analysis and low-energy loss spectroscopy. Ultramicroscopy 96 535-546. 

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. 

The major spectroscopic techniques for use in adhesive bonding technology that are based on vibrational principles are several forms of infrared spectroscopy, Raman spectroscopy, and the more recent technique HREELS, the vibrational version of EELS used in electron microscopes. These techniques will be discussed in this chapter and some recent developments and applications of the techniques in adhesion studies will be given. Raman and IR... 

Several recent overviews of principles and applications of Raman, FTIR, and HREELS spectroscopies are available in the literature [35-37, 124]. The use of all major surface and interface vibrational spectroscopies in adhesion studies has recently been reviewed [38]. Infrared spectroscopy is undoubtedly the most widely applied spectroscopic technique of all methods described in this chapter because so many different forms of the technique have been developed, each with its own specific applicability. Common to all vibrational techniques is the capability to detect functional groups, in contrast to the techniques discussed in Sec. III.A, which detect primarily elements. The techniques discussed here all are based in principle on the same mechanism, namely, when infrared radiation (or low-energy electrons as in HREELS) interacts with a sample, groups of atoms, not single elements, absorb energy at characteristic vibrations (frequencies). These absorptions are mainly used for qualitative identification of functional groups in the sample, but quantitative determinations are possible in many cases. 

Polystyrene. HREELS is usually applied in a qualitative way for the study of adsorbed molecules. The theoretical frame for these studies was established some 20 years ago [47]. However, its application to polymer surfaces is more recent [117, 118]. Polystyrene is a very common polymer and has been extensively studied in optical absorption spectroscopy and is often taken as a standard sample. It was extensively used as a standard sample to evidence the ability of HREELS to study polymeric surfaces, namely its quantitative capabilities. 

Ex Situ Methods XPS and HREELS will continne to be very usefnl ex sitn methods for the stndy of electrode surfaces. Soft X-ray XAS in both the EY and FY modes shonld find wider application. 

There is a number of vibrational spectroscopic techniques not directly applicable to the study of real catalysts but which are used with model surfaces, such as single crystals. These include reflection-absorption infrared spectroscopy (RAIRS or IRAS) high-resolution electron energy loss spectroscopy (HREELS, EELS) infrared ellipsometric spectroscopy. 

HREELS (Table 4.1) has the advantage of detecting all types of vibration this is because there are two excitation mechanisms, viz. dipole scattering and impact scattering. The former is subject to the same selection rules as RAIRS and gives strong features on-specular, but the latter excites all vibrational modes. There are however supplementary selection rules that apply to impact scattering in the on-specular direction. As noted earlier, this technique is not applicable to supported metal catalysts. 

Electron energy loss spectroscopy (EELS) and high resolution electron energy loss spectroscopy (HREELS) can also provide vibrational information of adsorbates on surfaces [165], Because these methods employ electrons instead of electromagnetic radiation, surface selection rules (see p. 76) are not effective this allows investigation of modes not observed with infrared spectroscopy. Unfortunately the use of electrons both as probe and signal prevents in situ application. Studies of electrode surfaces are feasible with these methods after emersion of the electrode from the solution, but they have been reported only infrequently. 

Among the related methods, specific experimental designs for applications are emphasized. As in-system synchrotron radiation photoelectron spectroscopy (SRPES) will be applied below for chemical analysis of electrochemically conditioned surfaces, this method will be presented first, followed by high-resolution electron energy loss spectroscopy (HREELS), photoelectron emission microscopy (PEEM), and X-ray emission spectroscopy (XES). The latter three methods are rather briefly presented due to the more singular results, discussed in Sections 2.4-2.6, that have been obtained with them. Although ultraviolet photoelectron spectroscopy (UPS) is an important method to determine band bendings and surface dipoles of semiconductors, the reader is referred to a rather recent article where all basic features of the method have been elaborated for the analysis of semiconductors [150]. 

The techniques highlighted here are XPS, AES, SIMS, various forms of FTIR, Raman spectroscopies, and HREELS. This selection is based on their relative ease of application and interpretation, their commercial availability, and the unique capabilities that each technique possesses for the study of an aspect of adhesive bonding. These capabilities are also highly complementary. The applications discussed are chosen to illustrate the applications in three major areas described earlier surface characterization, modification of metal or polymer surfaces, and analysis of interfaces. 

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