S-polarized light

On metals in particular, the dependence of the radiation absorption by surface species on the orientation of the electrical vector can be fiilly exploited by using one of the several polarization techniques developed over the past few decades [27, 28, 29 and 30], The idea behind all those approaches is to acquire the p-to-s polarized light intensity ratio during each single IR interferometer scan since the adsorbate only absorbs the p-polarized component, that spectral ratio provides absorbance infonnation for the surface species exclusively. Polarization-modulation mediods provide the added advantage of being able to discriminate between the signals due to adsorbates and those from gas or liquid molecules. Thanks to this, RAIRS data on species chemisorbed on metals have been successfidly acquired in situ under catalytic conditions [31], and even in electrochemical cells [32]. 

We are now in a position to calculate the reflections from multiple mterfaces using the simple example of a thin film of material of thickness d with refractive index n.2 sandwiched between a material of refractive index (where this is generally air witii n = ) deposited onto a substrate of refractive index [35, 36], This is depicted in figure Bl.26.9. The resulting reflectivities for p- and s-polarized light respectively are given by ... 

Fig. 4.55. Experimental and calculated (dashed line) RAIR-spectra for poly(methyl methacrylate) films 3270 + 100 nm, 362 30 nm, and 78 + 15 nm thick (a) p-polarized light incident at 60° (b) s-polarized light incident at 60°, after [4.266].

Fig. 10. The dependence of SH intensity in the mixed monolayer of C180AZ0N02 C1180C00H=1 1 on incident angle, a) and b) are p-polarized SH intensity when p- and s-polarized light was incident, respectively.

Although by now a large number of electrochemical systems have been examined using both SERS and IRRAS, including some common to both techniques (2b), the conditions employed are usually sufficiently different (e.g. disparate surface state, adsorbate concentrations) so to preclude a quantitative comparison of the spectral responses. One further hindrance to such comparisons is that it usually is difficult to remove entirely the contribution to the infrared spectra from solution-phase species. Two types of approaches are commonly used in IRRAS with this objective in mind. Firstly, modulating the infrared beam between s- and p-polarization can achieve a measure of demarcation between surface and bulk-phase components since considerably greater infrared absorption will occur for the former, but not the latter, species for p- versus s-polarized light (2.81. However, a complication is that the "surface... 

Polarization mechanisms, 10 21 Polarized ir radiation, 24 234 p-Polarized light, 24 115 s-Polarized light, 24 115... 

Solvent displacement, and isotherms. 954. 955 Solvent excess entropy at the interface, 912 Solvent interactions, 923, 964 Soriaga, M., 1103, 1146 Specifically adsorbed ions, 886 Spectrometer, 797 Spikes, electrodeposition. 1336 Spillover electrons, of metal, 889 Spiral growth, electrodeposition, 1316, 1324, 1326, 1324,1328 s-polarized light, 802 Srinivasan, S 1439,1494 Standard electrode potential American convention, 1354 convention, 1351 rUPAC convention, 1355 prediction of reactions, 1359 the zinc-minus and copper-plus convention, 1352... 

Figure 10. Photocurrent spectra for p-polarized light at different angles of incidence, normalized to s-polarized light, represented by the spectrum at 0° (GaSe electrode with N, = 7 X 1016 cm 3 at V3ce = —0.7 V electrolyte /M H,SOk)...

The presence of a thin film, or a stack of films, at an interface will affect the polarization properties of reflected and transmitted light. The analysis for isotropic materials is simplified by the fact that the Jones matrix will not contain off-diagonal elements. This means that the reflection and transmission of p and s polarized light can be treated separately and these subscripts can be dropped in the description of the components of the electric vector. 

When neither s-polarized light (light polarized perpendicular to the plane of incidence) is converted into p-polarized light (light polarized parallel to the plane of incidence) nor vice versa, standard SE is applied. This is the case for isotropic samples and for uniaxial samples in the special case, where the optical axis is parallel to the sample normal, for example (0001) ZnO [119]. 

Standard SE determines the complex ratio p of the reflection coefficients for p-polarized and s-polarized light... 

In the general case, when s-polarized light is converted into p-polarized light and/or vice versa, the standard SE approach is not adequate, because the off-diagonal elements of the reflection matrix r in the Jones matrix formalism are nonzero [114]. Generalized SE must be applied, for instance, to wurtzite-structure ZnO thin films, for which the c-axis is not parallel to the sample normal, i.e., (1120) ZnO thin films on (1102) sapphire [43,71]. Choosing a Cartesian coordinate system relative to the incident (Aj) and reflected plane waves ( > ), as shown in Fig. 3.4, the change of polarization upon reflection can be described by [117,120]... 

In the case that an organic phase contains light absorbing compounds, an external reflection (ER) absorption spectrometry is more useful than a TIR spectrometry [30,31]. Another advantage of the ER method is its higher sensitivity than the TIR method, especially as using s-polarized light. Therefore, it can be used as a universal absorption spectrometry of adsorbed species. Typical optical cells used for the TIR spectrometry and ER absorption spectrometry are shown in Fig. 3. 

Within the polarization scheme used here where p-polarized light is polarized in the plane of incidence, and s-polarized light is polarized perpendicular to the plane, the Cartesian components of , (a>, ) are given by the projection of the s and p components of the incident wave... 

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. 

Fig. 10. Typical spectral features for solution species, (a) Band for the asymmetric Cl-O vibration of Ci04 ions, measured with s-polarized light. Spectra calculated as the ratio between the smgle-beam spectrum at the sample potential (as indicated) and a spectrum at 0.05 V. (b) Integrated band intensity for the Cl-O stretching of perchlorate ions in solution as a function of potential.

Fig. 11. (a) Spectral features for sulfate species in solution, measured with s-polarized light at the indicated potentials reference spectrum taken at 0.05 V vs. RHE. Loss band at 1120cm asymmetric S-O stretch of SO4" gain band at 1190cm asymmetric stretch of HSO. Solution 0.5 M K2SO4+O.OI M H2SO4. (b) Integrated band intensities as a function of potential. 

The difference in spectral response for s- and p-polarized light has been used very often to discriminate between adsorbate and solution bands. This criterion has been criticized [27] on the basis that the intensity of the electric field vector for s-polarized light may be very low if the solution layer between electrode and IR window is very thin. This causes the spectrum obtained with s-polarized light to have a very low intensity. In this case the absence of bands with s-polarized light does not necessarily imply the absence of absorbing species in solution. 

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