Photon reflection-absorption infrared

Perhaps the first evidence for the breakdown of the Born-Oppenheimer approximation for adsorbates at metal surfaces arose from the study of infrared reflection-absorption line-widths of adsorbates on metals, a topic that has been reviewed by Hoffmann.17 In the simplest case, one considers the mechanism of vibrational relaxation operative for a diatomic molecule that has absorbed an infrared photon exciting it to its first vibrationally-excited state. Although the interpretation of spectral line-broadening experiments is always fraught with problems associated with distinguishing... 

Figure 13.2 Examples of infrared reflectivity from aortic atheromas. Typical intensity profiles of (a) the incoming synchrotron IR beam, (b) reflected signals from atheromas, and (c) from non-atherosclerotic sites, (d) The corresponding reflection-absorption spectra of (b) shared spectral characteristics that were consistent with known excitation effects by IR photons on atoms of molecules that are known to characterize atherosclerotic plaques. Each plot shows the averaged spectrum (black trace) 1.0 standard deviation (gray trace) n = 26.

Within catalysis and surface science, vibrational spectroscopy techniques are important tools to identify adsorbates, study their binding behavior, can provide information on adsorption sites. In the case of infrared spectroscopy, the most common form of vibrational spectroscopy, the vibrations in molecules are excited by the absorption of photons in the infrared range. Since the IR has only been used for few measurements within this work the experimental setup is described below but no theory is presented here, but can be found elsewhere [37, 101-104]. In order to measure infrared adsorption spectra on single crystal surfaces, a variation of IR spectroscopy is used, called infrared reflection adsorption spectroscopy (IRRAS). A schematic sketch of the IRRAS setup of the nanocat is depicted in Fig. 3.11. 

That is, the properties of liquid/solid or gas/Uquid interfaces cannot be investigated properly with XPS. On the other hand, in case of using photon-in and photon-out techniques, e.g., sum frequency generation (SFG) spectroscopy [15] or polarization modulation infrared reflection-absorption spectroscopy (PM-IRAS) [16, 17], it is possible to look into various surfaces and interfaces under elevated pressure condition or even reaction conditions, including liquid/solid interface. However, the interpretation of its result is often not straightforward, and it often requires the advanced understanding to extract information, e.g., the difficulty in quantitative analysis. 

The absorption or reflection by the solid sample of photons of various energies, from the ultraviolet to the infrared regions, are probably the most accessible and widely used techniques [8]. Absorption of infrared radiation by mulls or dispersions in alkali-halide discs are standard procedures. The sample holder may be capable of being heated so that peaks of interest can be monitored during the progress of decomposition. Hisatsune et al. [9] have successfully used infrared measmements to follow the decompositions of metal carboxylates (Chapter 16) incorporated in KBr discs. Spectra of powders may differ significantly from those of the same material in the form of larger crystals. 

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