S-polarized radiation

While s-polarized radiation approaches a phase change near 180° on reflection, the change in phase of the p-polarized light depends strongly on the angle of incidence [20]. Therefore, near the metal surface (in the order of the wavelength of IR) the s-polarized radiation is greatly diminished in intensity and the p-polarized is not [9]. This surface selection rule of metal surfaces results in an IR activity of adsorbed species only if Sfi/Sq 0 (/i = dipole moment, q = normal coordinate) for the vibrational mode perpendicular to the surface. 

The problem of solvent absorption can be overcome by measuring the change in reflectivity of the electrode either by (a) modulating the state of light polarization between p-polarized and s-polarized radiation, or (b) using p-polarized radiation and taking spectra at two different electrode potentials. 

Figures 4A and 4B present experimental spectra that illustrate the principle that the incoming E field distribution helps govern the type of spectra obtained. In Figure 4A, spectra of a DPPC monolayer are presented which were obtained at 60 angle of incidence with s-polarized radiation. As in previous studies where the experimental angle of incidence was 30° (2-6), the observed spectra have negative absorbances. In Figure 4B, however, the spectra of the monolayer taken with p-polarized radiation show positive absorbance bands, as predicted from theory (Figure 3).

Figure 3. Theoretical absorbance for a monolayer at the A/W interface plotted as a function of the experimental angle of incidence of the incoming radiation. The solid line indicates the theoretical absorbance for s-polarized radiation the dashed line indicates the theoretical absorbance for p-polarized radiation.

Figure 6.4-1 Notation used for Snell s law. The amplitude of the electric vector of the incident wave is q. r and t are the amplitude coefficients for reflection and transmis.sion, respectively. The electric vector of s-polarized radiation is perpendicular to the plane shown.

Figure 6.4-10 Reflectance spectra of poly (methyl methacrylate), PMMA, for s-polarized radiation. Angles of incidence indicated.

Fig. 4. Phase shifts upon reflection on a metal surface for p- and s-polarized radiation, (a) Incident and reflected field vector (b) phase shift as a function of the angle of incidence.

Figure 10 a and b shows the effect of applying an increasing positive potential to a plantinum electrode in HCIO4 solution. The integrated band intensities were taken from spectra measured with s-polarized radiation. The intensity of the band at 1100 cm , which is due to the asymmetric mode of CIO4 ions in the solution, follows the changes in H" concentration in the thin layer. 

Contrary to external reflection, where the field amplitude for s-polarized radiation is zero upon reflection, in the case of internal reflection the amplitude of the field at the reflecting surface is not zero for any direction of the field. The magnitude of the amplitude depends on the angle of incidence and on the difference of the refractive indices [33]. 

The sharp peak around 20° corresponds to the surface plasmon excitation, but this arises from the modeling and does not actually occur. The intensity for s-polarization is neghgibly small and thus omitted in the figure. The polarization dependence arises from the constructive and destructive interactions of multiple reflected p- and s-polarized radiations, respectively, within the metal layer [23]. 

Fig. 9. 3 Directions of the electric field vectors of the incident (dotted arrows) and the reflected (solid arrows) IR beams at the air/gold interface for (a) p-polarized and (b) s-polarized radiation.

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). 

Fignre 2.47 allows comparison of the spectra from DRIFTS and IRRAS of an organic monolayer (dodecylamine) on crystalline quartz in the region of the vCH vibrations. Optimum conditions for IRRAS (70° angle of incidence and s-polarized radiation) were chosen as described in Section 2.3.1. The resolution is higher in the spectrum collected by IRRAS, since the polished substrate surface is more uniform. However, the SNR is substantially higher in DRIFTS, which can be explained, in addition to the optical effects, by a higher surface density of the surfactant adsorbed on a fine powder than on a polished surface (Section 7.4.3). As can be concluded from the spectra reported in Ref. [174], the sensitivity threshold for the vCH bands in the DRIFTS is about 0.1 monolayer (ML). 

Figure 3.89. Calculated absorbances - log(fl/flo) in IRRAS spectra of hypothetical adsorbate vibration at 3000 cnr on silicon and glass surface as function of light incidence angle v>i for p-polarized radiation (Ap, dotted lines) and s-polarized radiation dashed lines). Solid lines denoted with and A represent parallel and perpendicular components, respectively, of total absorbance Ap] cpB is Brewster angle. Optical constants d2 = 1 nm, 02 = 1-5, k2 = 0.1, rigiass = 1.5, risi = 3.42. Reprinted, by permission, from H. Brunner, U. Mayer, and H. Hoffmann, Appl. Spectrosc. 51, 209 (1997), p. 211, Fig. 2. Copyright 1997 Society for Applied Spectroscopy.

The mean quadratic field, referred to the incident mean quadratic field in the same direction for the s-polarized radiation, is [6]... 

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