Angle of Incidence and the Thin-cavity Thickness

The previous section demonstrated that it is essential to control both the angle of incidence and the thin-cavity thickness in the IRRAS experiment. While it is usually not too difficult to control the angle of incidence, a measurement of the thin-cavity thickness with a precision of a fraction of a micrometer is not a trivial task. A new method [40] of determination of these important parameters is described in Fig. 9.12 for an experiment performed using Bap2 as the window, D2O as the solvent, and Au as the electrode. 

one measures the reflectivity Rq of an empty cell. In the absence of the electrolyte, the beam is totally reflected from the inner surface of the Bap2 

Determination of the isotropic Opticai Constants in Aqueous Soiutions 

One needs to know optical constants to calculate IRRAS spectra of molecules either adsorbed at the electrode surface or resident inside the thin-layer cavity. The isotropic optical constants of a given compound are usually determined from transmittance spectra. A pressed peUet, prepared by grinding the dispersion of the compound with a KBr or KCl powder, is typically used as a sample. Recently, Arnold et al. [41] have demonstrated that this method can yield non-reproducible results due to different histories of the sample preparation. In addition, the optical constants determined using the powder method can be quite different from those of the film at the metal/electrolyte interface because of the difference in the environment. 

we describe a procedure, adapted from the work of AUara et al. [42], which allows one to determine optical constants of a given compound in a solution. A thin-layer transmission optical cell, shown in Fig. 9.13 A, is employed to obtain the experimental data. Two ZnSe or, better, Bap2 discs are used as windows, and a 10- or 25-pm thick Teflon gasket is used as a spacer. The cell is placed in a Teflon housing, clamped between two aluminum plates, and mounted inside the main compartment of an FTIR spectrometer. Teflon tubing is used to fill the cell with fluid samples. Two samples are necessary to acquire a set of experimental spectra needed for calculations the pure solvent is used as a background sample and a solution of a given compound is used as the analyte sample. 

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Optimization of the Angle of Incidence and the Thin-cavity Thickness... 

The second method, developed by AUara et al. [42, 48, 49], relies on calculation of the theoretical reflection absorption spectrum for the same angle of incidence and the thin-cavity thickness as the values used during the collection of the experimental data. The optical constants of the window, electrolyte, and metal can be taken from the literature [22, 37-39], while the isotropic optical con-... 

Fig. 9.5 Mean square electric field strength at the metal surface for a p-polarized beam as a function of the angle of incidence and the thin.cavity (gap) thickness. Calculate for the convergent ( 6°) radiation of 1600 cm" . For stratified medium Cap2/D20/Au.

The main advantage of this method is that the absolute value of 0 can be determined directly. Howevei supplementary information such as the surface concentration, the angle of incidence, the thin-cavity thickness, and the optical constants of the film have to be determined from independent measurements. Below, we show several examples of how to apply this method. 

Changes in the composition of the solution in the thin layer cavity can be estimated in the order of 20% for the cases described in Sec. 4.3. For this condition it was assumed that the real part of the refraction index remains constant. AR/R was calculated as a function of the thickness of the thin layer of solution between electrode and IR window. The optical parameters (refractive index and absorption coefficient k ) used for the simmulation were Wwindow = 1 -4 solution = 1 -29, solution = 0-0348 Umetai = 8-9, /tmetai = 46. The results for two different wavelengths and two angles of incidence are shown in Fig. 12. 

Table 9.1 The refractive index, reflectance of the air/window interface (at normal incidence), the maximum MSEFS at the metal surface, the coordinates of the maximum (thin cavity thickness and the angle of incidence), the fuii width at half maximum (FWHM) of the MSEFS for different opticai window materials and the low frequency cut off limit.

Fig. 9.12 (a) Total reflection from the empty cell (b) reflection from the filled cell (c) reflectivity spectrum of the setup (dashed line) experimental values (solid line) calculated using Fresnel formulae for Bap2/D20/Au interface and the angle of incidence of 60°. Thin-cavity thickness was determined to be 2.4 pm. 

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