Infrared ER spectrometry

Here, the target of measurement by infrared ER spectrometry is a thin film deposited on a flat surface of a dielectric material. Multilayered adsorbed species (adsorbates) may also be a target, but, if a quantitative analysis of their observed spectra is to be performed, each layer in the adsorbate film should have a flat and parallel structure. For this reason, the discussion in this chapter assumes that thin films like a Langmuir-Blodgett (LB) film [4], which has an ideally flat layer, are the target of study. A dielectric substrate is made of a nonmetallic material and includes semiconductors the surface of water is also regarded as a dielectric substrate. Monolayers spread onto the surface of water are often analyzed by ER spectrometry. 

Infrared ER spectrometry depends on the reflection characteristics of differently polarized radiation at an interface. Figure 9.4 shows the calculated dependencies of the reflectances... 

The following discussion of infrared ER spectrometry is carried out using a theoretical framework of the three-phase system consisting of air/film/substrate. The substrate should have a polished fiat surface on the film side and a coarse surface on the other side. Otherwise, the system would have to be dealt with as a four-phase system consisting of air/film/substrate/air, which is totally different from a three-phase system. 

Infrared ER spectrometry has no mechanism of intensity enhancement, in contrast to other methods such as RA spectrometry to be described in Chapter 10 and surface-enhanced infrared absorption (SEIRA), mentioned in Chapter 13. Nonetheless, infrared ER spectrometry provides a unique technique for utilizing s- and p-polarized radiations for obtaining information governed by the surface selection rule on the transition dipoles of molecular vibrations. Theoretical analysis of the information obtained by this technique has the possibility for elucidating molecular orientations in thin films on dielectric substrates and molecular interactions in a wide variety of materials, including liquid crystals. 

If a material could be made extremely thin, for example, to the level of a single layer of molecules, this thin layer would transmit almost all of the infrared radiation, so that its infrared transmission spectrum could be measured. In fact, it is possible to measure a mid-infrared transmission spectrum from a thin soap film. It is usually practically difficult, however, to maintain such a thin film without it being supported by a substrate. For a thin film supported on a substrate, its infrared spectmm is often obtained by utilizing a reflection geometry. Two reflection methods are available for measuring infrared spectra from substrate-supported thin films, depending on the dielectric properties of the substrates used. External-reflection (ER) spectrometry, which is the subject of this chapter, is a technique for extracting useful information from thin films on dielectric (or nonmetallic) substrates, while reflection-absorption (RA) spectrometry, described in Chapter 10, is effective for thin films on metallic substrates [1]. In addition to these two reflection methods, attenuated total-reflection (ATR) spectrometry, described in Chapter 13 and emission spectroscopy, described in Chapter 15 may also be useful in some specific cases. 

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