Vibrational spectroscopies HREELS

NMR, EXAFS/XANES, and STM (see Section 2.4.2). Procedures specific to chemisorbed states 17.29,30,68-70 include measurement of changes in electrical or magnetic character, vibrational spectroscopies, (HREELS, DRIFTS/RAIRS etc), calorimetry and thermal desorption. This short list is far from being comprehensive, and the reader is reminded that the purpose of this work is not to instmct in the use of these techniques, but rather to present and evaluate the results they generate. The principles concerned are only mentioned where understanding of the results necessitates it. This is somewhat in the spirit of Jerome K. Jerome, who in his Preface to Three Men in a Boat (to say nothing of the dog) advised his readers not to use it as a manual for a River Thames holiday they would, he said, be wiser to stay ay home. 

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. 

In this chapter, three methods for measuring the frequencies of the vibrations of chemical bonds between atoms in solids are discussed. Two of them, Fourier Transform Infrared Spectroscopy, FTIR, and Raman Spectroscopy, use infrared (IR) radiation as the probe. The third, High-Resolution Electron Enetgy-Loss Spectroscopy, HREELS, uses electron impact. The fourth technique. Nuclear Magnetic Resonance, NMR, is physically unrelated to the other three, involving transitions between different spin states of the atomic nucleus instead of bond vibrational states, but is included here because it provides somewhat similar information on the local bonding arrangement around an atom. 

W. H. Weinberg. In Methods of Experimental Physics. 22,23, 1985. Fundamentals of HREELS and comparisons to other vibrational spectroscopies. 

Vibrational Spectroscopy of Molecules on Surfaces. 0- T. Yates, Jr. and T. E. Madey, eds.) Plenum, New York, 1987. Basic concepts and experimental methods used to measure vibrational spectra of surface species. Of particular interest is Chapter 6 by N. Avery on HREELS. 

Vibrational spectroscopy provides the most definitive means of identifying the surface species arising from molecular adsorption and the species generated by surface reaction, and the two techniques that are routinely used for vibrational studies of molecules on surfaces are Infrared (IR) Spectroscopy and Electron Energy Loss Spectroscopy (HREELS) (q.v.). 

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. 

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]).

A versatile tool to analyze vibrations of surface atoms and adsorbed molecules is high-resolution electron energy loss spectroscopy (HREELS) [359], Monoenergetic low-energy electrons (1-10 eV) are directed to the surface. Most of them are backscattered elastically. 

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. 

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. 

The study of polymer surfaces would certainly greatly benefit from HREELS contributions, electron-induced vibrational spectroscopy being often presented as the surface counterpart of the classical optical infra-red and Raman spectroscopies. However, the first HREEL spectra from polymer surfaces were published only in 1985 for thin organic films (2), in 1986 for a real insulating thick polyethylene... 

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. 

In the case of molecules adsorbed at surfaces, it must be first stated that much important information is obtained from high-resolution electron energy loss spectroscopy (HREELS). This technique measures vibration frequencies of surfaces, in a way similar to infra-red absorption spectroscopy in the gas phase. HREELS allows the identification of the molecular species present on the surface, which no surface crystallography method can do. 

In order to show that the strongly bound species was actually an EpB molecule, high-resolution electron energy loss spectroscopy (HREELS) was used to study the species present at the various dosing temperatures. When dosed at lower temperatures, most of the observed peaks in the HREELS matched those of the vibrational spectrum of liquid EpB, suggesting that intact EpB is interacting with the silver surface at lower temperatures. However, the silver surface dosed with EpB at 300 K showed noticeable differences in the HREELS spectrum. In addition, DPT calculated vibrational frequencies of the surface bound oxaraetallacylce matched well with those determined experimentally. 

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