P-Polarized radiation

With p-polarized radiation and incident angles near grazing incidence an increase in sensitivity of approximately a factor of 25 can be achieved in comparison with transmission experiments [4.265]. This advantage is reduced to a factor of 17 for a more realistic experimental situation in which the spread of incident angles is ca. 5° at approximately 85°. 

Fig. 4.54. IR reflection spectra from an ODS monolayer on silicon for s- and p-polarized radiation at different incident angles ft Symmetric (s), asym-...

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. 

Relation (3.1.25) for the integral intensity of the /th spectral line for s- and p-polarized radiation is conveniently expressed in terms of polarization vectors (3.3.3) ... 

Fig. 4.2. Diagrammatic representation of the experimental crystal and beam geometries for p-polarized radiation incident upon the (111) crystal face as viewed (a) from the side and (b) from the top including the second atomic plane ( ). The crystal coordinates are labeled x, y, z with the Z direction along the [211] crystal direction. The beam coordinates are labeled s, k, z. From Ref. 122.

In addition to describing the conformation of the hydrocarbon chains for amphiphilic molecules at the A/W interface, external reflectance infrared spectroscopy is also capable of describing the orientation of the acyl chains in these monolayers as a function of the monolayer surface pressure. The analysis of the orientation distribution for an infrared dipole moment at the A/W interface proceeds based on classical electromagnetic theory of stratified layers (2). In particular, when parallel polarized radiation interacts with the A/W interface, the resultant standing electric field has contributions from both the z component of the p-polarized radiation normal to the interface, as well as the x component of the p-polarized radiation in the plane of the interface. The E field distribution for these two... 

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.

The high-wavenumber limit of such a reflectance band is clearly defined by the point where n equals unity v 1090 cm ), i.e. the refractive index of ambient air. The other occurrence of n = 1 close to o is accompanied by such high absorption, that considerable reflectance is observed at least for the s-polarized component. The reflectance band for p-polarized radiation is limited to short wavenumbers where the condition for the Brewster angle is met. This condition is met a second time within the reflection band. This does not become evident in the reflectance spectrum due to the related high value of k. 

Figure 6.4-12 Measured reflectance of p-polarized radiation close to the Brewster angle of polyfmethyl methacrylate).

In practice, s- and p-polarized radiation are used to discriminate between spectral features due to absorbates or solution species we shall return to this subject in Sec. 4.4. 

The technique of subtracting spectra taken with linearly s- and p-polarized radiation is based on the surface selection rule for reflection-absorption on a metal surface [20] ... 

Therefore, only p-polarized radiation which has an electric field vector parallel to the plane of incidence can interact with absorbed species at the surface. On the contrary, s-polarized light, having the electric field vector perpendicular to the plane of incidence, can interact only with species in solution. 

This result could explain differences in spectral data in the same systems obtained with low incidence angles (flat window) and high incidence angles (prismatic or spherical windows). The latter can furnish almost featureless spectra for solution species with s-polarized light (compare the curves in Fig. 12 at rf = 1 pm) and induce the wrong conclusion that no solution feature interferes in the spectra measured with p-polarized radiation [35-37]. On the contrary, the use of a flat window leads to comparable intensities for s- and p-polarized radiation spectra [27,38]. 

The adsorption of cyanide on Pd electrodes was studied by using a combination of polarization and potential modulation (FT-IRRAS and SNIFTIRS) [124]. The reason for this combination is to enhance the surface signal, since two FT-IRRAS spectra taken at two different potentials were ratioed to obtain the SNIFTIRS spectrum. Despite this effort, a solution band at 2135 cm" (also observed with s-polar-ized light) persists in the spectrum (Fig. 36). The spectrum in this Figure taken with p-polarized radiation presents, however, a strong band at 1980 cm", which was attributed to a bridged-bonded cyanide ion, and a weak band at 2065 cm" assigned to linearly adsorbed C-down CN". 

FIG. 4 FT-IRRAS spectrum in midfrequency region for gold substrate treated with 5 -TsTsTsTsTsTCTCATACATG-3. The p-polarized radiation was introduced on the sample at 85° off the surface normal and data were collected at a spectral resolution of 4 cm-1 with 256 scans. 

This plot shows that the maximum sensitivity is obtained by using p-polarized radiation at a high incident angle around 80°. The use of the high incident angle is also favorable for reducing the absorption by the solution due to the decrease in the penetration depth of the evanescent wave. Nevertheless, two issues should be noted about the optical conditions in the practical measurements. 

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