Reflection absorption

The term reflection absorption classes a group of experiments where the sample to be analyzed is placed on a mirror so that the absorption of the sample reduces the reflectance, and a sort of transmission spectrum of the sample results. The sample thickness ranges from many times the wavelength down to fractions of a monolayer. At either end of this scale special experimental modifications provide improved sensitivity with respect to the actual thickness. 

With a thickness below quarterwave, the response decays not only with the thickness but additionally with the field amplitude approaching the node. Thus zero sensitivity is to be expected at the very surface on the metal substrate. Advantageously another ex- 

In the extreme case of grazing incidence, a field component exists only normal to the surface. Therefore an interaction is possible exclusively with transition moments or components thereof, orientated perpendicular to the surface. This anisotropy of interaction can also be explained by selection rules, which are based on symmetry consideration and include the mirror image of the analyte produced by the metallic surface. 

The resulting spectral features are comparable to absorption bands, their amplitude being very small in comparison to the reflected intensity. Therefore it is essential for the measurements, that due to destructive interference the perpendicular component of incident radiation shows hardly any interaction with the absorbing surface species. Therefore 

The underlying optical phenomena are quite complex and the experimental results cannot be anticipated straightaway. Therefore simulations are generally necessary to discriminate shifts and distortions of spectral band shapes caused by optical effects from those caused by surface-induced changes in structure or chemical bonding (Allara et al., 1978 Porter, 1988). 

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Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

Szanyi J, Kuhn W K and Goodman D W 1994 CO oxidation on palladium 2. A combined kinetic-infrared reflection absorption spectroscopic study of Pd(IOO) J. Phys. Chem. 98 2978... 

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

Attenuated total reflectance spectroscopy and reflection-absorption infrared spectroscopy... 

Whereas ATR spectroscopy is most commonly applied in obtaining infrared absorption spectra of opaque materials, reflection-absorption infrared spectroscopy (RAIRS) is usually used to obtain the absorption spectrum of a thin layer of material adsorbed on an opaque metal surface. An example would be carbon monoxide adsorbed on copper. The metal surface may be either in the form of a film or, of greaf imporfance in fhe sfudy of cafalysfs, one of fhe parficular crysfal faces of fhe mefal. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

In order to characterize the surface regions of a sample that has been modified in some way, as is usually the case in adhesion-related investigations, some sort of a reflection experiment is required. Two types of experiments, attenuated total reflection (ATR) and reflection-absorption infrared spectroscopy (RAIR),... 

Fig. 4. Schematic drawing of a reflection-absorption infrared (RAIR) spectroscopy experiment.

If the entry of a molecule into the body were simply a temporally restricted absorption process, then a steady-state concentration would be achieved given enough time for complete absorption. However, what in fact is observed in drug pharmacokinetics is a complex curve reflecting absorption of the drug into the body and the diminution of the concentration that is absorbed back down to negligible levels. The reason for this complex pattern of rise and fall in... 

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