External Reflection Metallic Substrates

The angular dependences of the MSEFs in a film at the air-Al and water-Al interfaces shown in Fig. 1.17a are remarkable in three respects. First, independently of the immersion medium, the z-component of the electric field within the film is dominant, while the x- and y-components are almost zero at aU angles of incidence (p. In other words, the tangential electric fields are quenched. This is observed for all metals. The second feature, which is common to all substrates in air (e.g., compare with Fig. 1.15a), is the attenuation of the perpendicular MSEF component, whose maximum value for Al is 0.73. It can be shown [161] that such an attenuation of the (El)-component in the case of a metal substrate is observed for films with 2 1-4, which includes the majority of films. Finally, it should be noted that if the radiation is incident fi om water onto a metal substrate, the perpendicular MSEFs within the film are enhanced by a factor of about 2, as for Si substrates (Fig. 1.15a). 

The distribution of the electric fields along z-axis in the air-model film-Al system is shown in Fig. 1.16b. The standing-wave patterns produced by the tangential electric field components exhibit nodes at a metal surface, while the normal component has an antinode. As seen from the insert in Fig. 1.16i , the tangential electric fields, which are continuous at interfaces (1.8.8°), decay dramatically after crossing the metal surface, typically at a distance similar to the depth of the skin layer (1.3.14°). 

ABSORPTION AND REFLECTION OF INFRARED RADIATION BY ULTRATHIN FILMS 

External electric field Induced electric field 

However, in the presence of a thin film at the air-metal interface, there is no enhancement of ( ) at the metal surface, as might be concluded based on the above consideration. The reason will be apparent if one analyzes the boundary conditions for the electromagnetic wave (1.4.7°). The continuity of the tangential components of the electric field and the perpendicular component of the electric displacement, D = eE, can be approximated for a three-phase system as [99, 101] 

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The second method (B) used is based on an equation for external reflection spectroscopy on metal substrates given in the paper of Qiabal [12], covering both IRRAS spectra and ATR of thin films on highly reflective metal substrates ... 

For many years, IRRAS has been successfully applied to the study of thin films adsorbates on metal surfaces [36], In the case of monolayers deposited on metal surfaces, an IR external reflection spectrum is obtained by reflecting the incoming radiation from the three-phase ambient-adsorbate-substrate system, measuring the reflected intensity as a function of wavelength, and then ratioing... 

Let us compare the thin-film approximation formulas for (1) the transmissivity (1.98) (2) the reflectivities for the external reflection from this film deposited onto a metallic substrate (1.82) (3) the internal reflection at (p > (pc (1.84) and (4) the external reflection from this film deposited on a transparent substrate (dielectric or semiconducting) (1.81) (Table 1.2). In all cases s-polarized radiation is absorbed at the frequencies of the maxima of Im( 2), vroi (1.1.18°), whereas the jo-polarized external reflection spectrum of a layer on a metallic substrate is influenced only by the LO energy loss function Im(l/ 2) (1.1.19°). The /7-polarized internal and external reflection spectra of a layer on a transparent substrate has maxima at vro as well as at vlq. Such a polarization-dependent behavior of an IR spectrum of a thin film is manifestation of the optical effect (Section 3.1). 

From the description of the setup, it is clear that techniques working with external reflection require reflecting electrodes. The major type used is bulk metal electrodes, either as polycrys-talline material or as single crystals. However, other reflecting electrode materials like glassy carbon may be used as well. These electrodes may either serve as system under smdy or simply as substrates, onto which the sample of interest (e.g., a conducting polymer) is deposited. Approaches where nanoparticles or carbon supported catalysts are deposited onto such electrodes and investigated with IR spectroelectrochemistry have been described as well (e.g., [11]). 

With optically transparent electrodes (OTE), molecular adsorbates, polymer films, or other modifying layers attached to the electrode surface or being present in the phase adjacent to the electrode can be studied. With opaque electrode materials, internal or external reflection may be applied. Glass, quartz, or plastic substrates coated with a thin layer of semiconductors (indium-doped tin oxide) or conducting metals (gold, platinum) are often used as OTE. The optically transparent electrode is immersed as working electrode in a standard cuvette. 

In addition to this chapter and Chapter 13, reflection-related measurements are described in Chapter 9 (external reflection measurements for thin films and interfaces). Chapter 10 (reflection-absorption measurements for thin films on metal substrates), and Chapter 12 (diffuse reflection measurements). 

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. 

In contrast to flowcell interfaces, solvent-elimination approaches lead to spectra free of solvent interferences. Various sampling techniques are possible the sample can be deposited on a flat ZnSe plate, on a smooth metallic substrate or a thin layer of powdered alkali halide salt, whereas the spectrum can be taken in conventional transmission, external-reflection or diffuse-reflection arrangement. In one of the first applications a synthetic mixture of three quinones was separated on a microbore column packed with silica, using a mobile phase of 5% methanol in supercritical carbon dioxide. The peaks were collected on a plate on which a layer of KCl powder was deposited, and then spectra were measured by a diffuse-reflection accessory. Test measurements on acenaphthenequi-none (AQ) showed later that conventional transmission spectra of samples on flat infrared-transmitting windows give the best compromise between high sensitivity, correct relative band intensities and adherence to the Lambert-Beer law. 

Infrared external reflection spectroscopy is used extensively in the study of thin films on metallic substrates, the incident beam is directly reflected from the metallic surface, and the transmitted beam is attenuated in the regions of substrate absorption for other materials. The reflected beam interacts with molecules situated at the surface of partially transmitting materials. The resulting reflectance spectra are functions of polarization and incidence angle of the incident beam and provide a quantitative measure of the surface concentrations and an indirect measure of the structure and orientation of molecules in the surface layer. 

Due to its very nature, the electrode/electrolyte interface may conveniently be studied by reflection-absorption spectroscopy. The first attempts in the infrared wavelength range were made with internal reflection spectroscopy. This allows multiple reflections at the electrode surface to increase the signal, which was otherwise too weak for direct measurement.However, due to inherent difficulties of this method (e.g., the need for a transparent substrate, the necessity for a thin metal layer as electrode), specular external reflection spectroscopy now is preferred for the in situ investigation of electrode processes. 

The terms FD C, D C, and External D C (Ext. D C), which are part of the name of colorants, reflect the FDA s colorant certification. FD C dyes may be used for foods, dmgs, and cosmetics D C dyes are allowed in dmgs and cosmetics and Ext. D C dyes are permitted only in topical products. Straight colorants include both the organic dyes and corresponding lakes, made by extending the colorant on a substrate such as aluminum hydroxide or barium sulfate. The pure dye content of these lakes varies from 2 to 80% the organic dyes contain over 80% pure dye. Colorants certified for cosmetic use may not contain more than 0.002% of lead, not more than 0.0002% of arsenic, and not more than 0.003% of heavy metals other than lead and arsenic. 

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