Adsorbate effects surface sensitivity

The sensitivity limitations of TLC-FT-Raman spectroscopy may be overcome by applying the SERS effect [782]. Unlike infrared, a major gain in Raman signal can be achieved by utilising surface activation and/or resonance effects. Surface-enhanced Raman (SER) spectra can be observed for compounds adsorbed on (rough) metahic surfaces, usually silver or gold colloids [783,784], while resonance Raman (RR) spectra... 

Therefore Z)opt provides a convenient single parameter that can be used to estimate the response of guided modes to very thin adsorbed layers. To assess and compare surface sensitivity, in this chapter, we will use the differential change of mode effective index with the optical thickness ... 

This chapter deals with the study of structural properties of catalysts and catalytic model surfaces by means of interference effects in scattered radiation. X-ray diffraction is one of the oldest and most frequently applied techniques in catalyst characterization. It is used to identify crystalline phases inside catalysts by means of lattice structural parameters, and to obtain an indication of particle size. Low energy electron diffraction is the surface sensitive analog of XRD, which, however, is only applicable to single crystal surfaces. LEED reveals the structure of surfaces and of ordered adsorbate layers. Both XRD and LEED depend on the constructive interference of radiation that is scattered by relatively large parts of the sample. As a consequence, these techniques require long-range order. 

Most of the contemporary research areas that utilize surface chemistry techniques employ thin organic films that have been physically or chemically adsorbed onto a solid (usually metallic) substrate. The use of conducting metal surfaces is due not only to their relevance to many different fields, but also to the fact that many surface spectroscopic techniques (Table I) need such a surface in order to produce high-quality spectra. This criterion effectively eliminated many interesting non-metallic surfaces from study using modem surface-sensitive spectroscopic methods. 

Adsorbate effects on a-Si H conductance have been shown to be of great interest because of the high sensitivity exhibited by the a-Si H surface to particular gases under appropriate conditions, although both stability and selectivity problems are still far from being solved. 

The UME current response becomes increasingly sensitive to the surface process as the tip/substrate separation is decreased, since this serves both to maximize the ratio of effective surface area to solution volume probed by SECMID and to hinder solution diffusion in the gap. This effect is illustrated by Figures 6a and b for various values of Ka and an adsorption/desorption system characterized by KJKd = 1 and A = 8 for L = 0.1 and 1.0, respectively. The closer the tip is positioned with respect to the substrate, the greater the depletion of the solution component of the adsorbate adjacent to... 

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