Infrared reflection adsorption spectroscopy

Support effects in electiocatalysis, 567-586 Surface diffusion, 163, 173-177 Surface Enhanced Infrared Reflection Adsorption Spectroscopy (SEIRAS), 183... 

This approach has the potential to resolve the time evolution of reactions at the surface and to capture short-lived reaction intermediates. As illustrated in Figure 3.23, a typical pump-probe approach uses surface- and molecule-specific spectroscopies. An intense femtosecond laser pulse, the pump pulse, starts a reaction of adsorbed molecules at a surface. The resulting changes in the electronic or vibrational properties of the adsorbate-substrate complex are monitored at later times by a second ultrashort probe pulse. This probe beam can exploit a wide range of spectroscopic techniques, including IR spectroscopy, SHG and infrared reflection-adsorption spectroscopy (IRAS). 

Figure 3.23 Schematic illustration of the femtosecond pump-probe technique used to initiate and follow the reactions of adsorbed species SHG, second harmonic generation SFG, sum frequency generation IRAS, infrared reflection-adsorption spectroscopy PPE, photons per event A R, change in reflectivity...

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. 

Xu, X.P. Vesecky, S.M. Goodman, D.W. Infrared reflection-adsorption spectroscopy and STM studies of model silica-supported copper catalysts. Science 1992, 258, 788. 

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. 

Application of advanced microscopic and spectroscopic methods, such as Brewster Angle Microscopy (BAM), Infrared Reflection-Adsorption Spectroscopy (IRRAS) and Grazing Incidence X-ray Dififaction (GIXD) at the monolayer revealed that when two lipid phases coexist, only the disordered liqnid-expanded phase is affected by the ions. Table I snm-marises GIXD results for the crystal cell parameters of the ordered liquid-condensed phase. These results prove that the ordered DPPC phase is not strongly affected by salts even at high salt concentrations. 

Islam, M.N., Okano, T., Kato, T. Surface phase behavior of a mixed system of anionic — nonionic surfactants studied by Brewster angle microscopy and polarization modulation infrared reflection — adsorption spectroscopy. Langmuir 2002, 78(26), 10068-10074. 

While the orientations evident from x-ray diffraction are not singularly specific for each monolayer, the x-ray data clearly indicate that the orientation of the crystals depends upon the chemical nature of the terminal functional group of the monolayer. This result was also confirmed using grazing angle infrared reflection adsorption spectroscopy (IRRAS).(79) It is reasonable to suggest that this behavior is a consequence of interfacid interactions during the nucleation process. 

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