External Reflection Transparent Substrates

It can be seen in Figs. 1.15a and b that for the model ultrathin organic film at the air-Si and water-Si interfaces aU components of the MSEEs within this film for all angles of incidence are weakened relative to the incident radiation ( () = 1). Comparison of the electric field intensities in the air and water environments reveals that the MSEF magnitudes within the film, (El), increase as the optical density of the surroundings ( i) increases. Deviation of the values of (El) from unity and their dependence on the optical parameters of the medium are due to the formation of interference patterns (standing waves) in 

ABSORPTION AND REFLECTION OF INFRARED RADIATION BY ULTRATHIN FILMS 

An important observation from Fig. 1.15 is that the TDMs with all spatial components are active in the spectra The y-components arise in the -polarized spectra, while the x- and z-components arise in the p-polarized ones. Moreover, as seen from Fig. 3.89, the x- and z-components in the j -polarized spectra are characterized by the differently directed absorption bands. This regularity constitutes the surface selection rule (SSR) for dielectrics (see Section 3.11.4 for more detail). 

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In 1960, Harrick demonstrated that, for transparent substrates, absorption spectra of adsorbed layers could be obtained using internal reflection [42]. By cutting the sample in a specific trapezoidal shape, the IR beam can be made to enter tlirough one end, bounce internally a number of times from the flat parallel edges, and exit the other end without any losses, leading to high adsorption coeflScients for the species adsorbed on the external surfaces of the plate (Irigher than in the case of external reflection) [24]. This is the basis for the ATR teclmique. 

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). 

External reflection, (semi)transparent substrate l/ Equation (1.81)... 

Both causes of spectral enhancement act in the case of external reflection from transparent or low-absorbing substrates. The positive geometric effect is demonstrated in Fig. 1.15. The negative effect due to the spectrum representation is the most pronounced in the p-polarized spectra measured at [Pg.55]

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. 

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. 

Two main methods were adopted in the past nine years. In the so-called external infrared reflectance spectroscopy method, the beam passes through a window, crosses a very thin layer of the electrolytic solution, and makes a single reflection at the electrode surface (Figure 6a). In the internal infrared reflectance spectroscopy method, the beam reaches the surface from the inside, i.e., through an optically transparent substrate on which the thin-layer electrode is deposited, and makes one or several reflections in order to improve the signal-to-noise ratio (Figure 6b). 

Internal reflection avoids the complication of radiation absorption by the solvent. However, the problem of solubility of the various materials in contact with the solution remains as acute as for external reflection. Furthermore, the material which constitutes the working electrode is generally opaque to the infrared radiation. It is therefore necessary to reduce the thickness to a very thin film deposited on the surface of an infrared-transparent substrate. 

A substrate 10 of HgCdTe is provided with an upper surface region 11 formed by an annealing procedure, or as an epitaxial layer or evaporated film. A layer of insulating material 12 is formed in which windows are provided. The windows are partially filled with a thin layer of metal 13 which is deposited therein to form a metal-semiconductor diode with the upper surface region. The metal layer is deposited to a thickness on the order of 10-50 nm thick and is sufficiently thin to be semi-transparent to infrared radiation. A thick layer of metal 14 is deposited to form an expanded contact and an anti-reflection coating 15 is provided. External conductors in the form of jumper wires 16 are ball bonded to the contact 14. 

Where the substrate is transparent, IR spectra of the surface layers can be obtained either by transmission or by reflection when the substrate is nontransmitting, then reflection is normally used. Specialised IR techniques that are suitable for surface analysis are external, internal and diffuse reflectance. When light propagating in a medium of refractive index t 2 reaches a medium of refractive index ni radiation is partly reflected and partly refracted and both parts contain information on the material composition. The ratio reflected/reffacted radiation depends on ni, 112 and the angle of incidence (0). The choice of methods for obtaining spectra by reflection has expanded significantly in... 

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