Electron energy loss spectroscopy information obtained

The vibrations of molecular bonds provide insight into bonding and stmcture. This information can be obtained by infrared spectroscopy (IRS), laser Raman spectroscopy, or electron energy loss spectroscopy (EELS). IRS and EELS have provided a wealth of data about the stmcture of catalysts and the bonding of adsorbates. IRS has also been used under reaction conditions to follow the dynamics of adsorbed reactants, intermediates, and products. Raman spectroscopy has provided exciting information about the precursors involved in the synthesis of catalysts and the stmcture of adsorbates present on catalyst and electrode surfaces. 

There are two types of electron energy loss spectroscopy currently in use. The first of these is found in scanning transmission electron microscopes. As indicated in Figure 5.1, compositional information may be obtained in the TEM by measuring the energy loss of the inelastically scattered electrons transmitted through a thin specimen. 

Electron energy-loss spectroscopy (EELS) is nowadays widely used to obtain the information with respect to chemical composition, oxidation state and electronic structure of solids. Since all catalytic processes concern the exchange of electrons between the reactants, EELS is extremely valuable in catalysts investigations [9, 49-57], EELS in an electron microscope exhibits the advantage of high spatial resolution in area of interests with simultaneous structure determination by electron diffraction and imaging. 

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. 

Experimental data can be obtained by ultra-violet absorption spectroscopy, electron energy loss spectroscopy and photoelectron spectroscopy. UV absorption and EELs have been described briefly in Chapter 3. The former provides information only about the band-gap, while EELs gives more general information about the conduction bands. Both X-rays and UV photons can be used to generate photoelectrons these two methods are given the acronyms XPS and UPS. The energy spectrum of the emitted electrons provides information about the density of electron states in the valence bands. In principle the size of the band gap can be obtained, but care must be taken as the absolute energy... 

Vibrational spectroscopy is an important probe used to determine the bonding and structural properties of molecules. Powerful techniques such as electron energy loss spectroscopy (EELS) have been developed, which allow one to obtain the vibrational properties of molecules chemisorbed upon surfaces. Due to low concentration, the highly reactive nature of the clusters, and the large number of possible species which are typically present in the cluster beams used to date, unconventional techniques are required in order to obtain spectroscopic information. One unconventional but powerful technique, infrared multiple photon dissociation (IRMPD), has recently been applied to the study of the vibrational properties of gas-phase metal clusters upon which one or more molecules have been chemisorbed. This same technique, IRMPD, has previously been used to obtain the vibrational spectra of ions, species for which it is difficult to apply conventional absorption techniques. 

In many materials problems, for example at surfaces or interfaces, the chemical composition and nuclear coordinates are not fully known. Indeed, any information which can be obtained by theory on these basic structural properties will be useful, in conjunction with experiment. Spatially Resolved Electron Energy Loss Spectroscopy (SREELS), X-ray near-edge absorption (XANES) and emission, Mossbauer spectra, etc. provide site-specific probes which can be combined with theory to help resolve structures. 

The most widely used technique to get information on the electronic structure of clean surfaces, nanostructures on surfaces, or even molecules adsorbed on surfaces is ultraviolet photoelectron spectroscopy (UPS). The difficulty of this method, when applying it to clusters on surfaces, is to obtain sufficient spectral contrast between the low number of adsorbed clusters and the substrate [45]. Thus, electron energy loss spectroscopy (EELS) is more successfully used as a tool for the investigation of the electronic structure of supported clusters. An interesting test case for its suitability is the characterization of supported monomers, i.e., single Cu atoms on an MgO support material [200], as this system has been studied in detail before with various surface science techniques [201-204]. The adsorption site of Cu on MgO(lOO) is predicted... 

From examining the spectroscopic characteristics of diamond films, many insights into their structure may be obtained. Most of all the Raman spectroscopy, XRD and electron energy loss spectroscopy (EELS) provide valuable information. Other methods like IR-spectroscopy and XPS shed light on the surface structure. These techniques are supplemented by microscopy methods, for example, by AFM and STM, so altogether the morphology of the films surface can be studied in quite some detail. 

Vibrational spectroscopy provides detailed information on both structure and dynamics of molecular species. Infrared (IR) and Raman spectroscopy are the most commonly used methods, and will be covered in detail in this chapter. There exist other methods to obtain vibrational spectra, but those are somewhat more specialized and used less often. They are discussed in other chapters, and include inelastic neutron scattering (INS), helium atom scattering, electron energy loss spectroscopy (EELS), photoelectron spectroscopy, among others. 

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