External reflectance

The electrochemical cell design employed in thin-layer reflection-absorption studies is shown in Fig. 20. 

The technique developed by Bewick for use with a dispersive IR spectrometer is termed electrically modulated infrared spectroscopy (EMIRS) and is essentially a direct development of the UV-VIS specular reflectance technique [50], modulated specular reflectance spectroscopy (MSRS). As in MSRS, radiation is specularly reflected form a polished electrode surface while the electrode potential is modulated with a square wave between a base potential and the working potential at which the process of interest occurs the wavelength range of interest is then slowly scanned. Only that 

Normalized absorbance vs. thickness plot for a layer of water in contact with a platinum reflector measured using an infrared beam at the wavelength of the maximum absorptin of the O-H stretching mode and incident at an angle of 45°, calculated using (a) Beer s Law and (b) the full Fresnel reflectance equations. 

This intensity difference, AR, observed in an EMIRS experiment may result from several sources, e.g. electroreflectance effects arising from changing electron densities at the surface of an electrode under the influence of the applied potential, changes in the amount of adsorbed species, or re-organisation of the double layer, etc. Spectra arising from changes in the adsorbed layer may be derived from 

Because an EMIRS experiment involves a relatively fast potential mo- 

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The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

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. 

Phosphoric acid ester was used as a model for the estimation of concentration of a reagent in an adsorbed layer by optical measurements of the intensity of a beam reflecting externally from the liquid-liquid interface. The refractive index of an adsorbed layer between water and organic solution phases was measured through an external reflection method with a polarized incident laser beam to estimate the concentration of a surfactant at the interface. Variation of the interfacial concentration with the bulk concentration estimated on phosphoric acid ester in heptane and water system from the optical method agreed with the results determined from the interfacial tension measurements... 

Reflectance spectroscopy in the infrared and visible ultraviolet regions provides information on electronic states in the interphase. The external reflectance spectroscopy of the pure metal electrode at a variable potential (in the region of the minimal faradaic current) is also termed electroreflectance . Its importance at present is decreased by the fact that no satisfactory theory has so far been developed. The application of reflectance spectroscopy in the ultraviolet and visible regions is based on a study of the electronic spectra of adsorbed substances and oxide films on electrodes. 

External reflectance. The most commonly applied in situ IR techniques involve the external reflectance approach. These methods seek to minimise the strong solvent absorption by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The result is a thin layer of electrolyte trapped between electrode and window usually 1 to 50 pm. A typical thin layer cell is shown in Figure 2.40. 

Figure 2.40 Schematic representation of the external reflectance cell design commonly employed in in situ IR experiments, if the working electrode is a semiconductor, then the semiconductor/ electrolyte interface can be studied under illumination with, for example, UV light by directing the beam perpendicular to the IR beam, as shown.

In employing a thin-layer configuration the external reflectance approach reduces the problem of the strong solvent absorption in two ways. Firstly, this configuration yields a solution layer only a few microns thick. Secondly, exact calculations employing the Fresnel reflection equations show that the radiation absorbed by an aqueous layer c. 1 urn thick in contact with a reflective electrode is attenuated to a lesser extent than would be predicted by the Beer - Lambert law. 

The three most commonly applied external reflectance techniques can be considered in terms of the means employed to overcome the sensitivity problem. Both electrically modulated infrared spectroscopy (EMIRS) and in situ FTIR use potential modulation while polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) takes advantage of the surface selection rule to enhance surface sensitivity. 

Electrically modulated infrared spectroscopy, ( EMIRS). In all three external reflectance approaches the signal processing technique serves two purposes (a) to remove the contributions to the reflected ray that do not change, e.g. the detector response, the source emission envelope, the solvent,... 

The in-sifu FTIR approach can be divided into the transmittance, external reflectance and internal reflectance methods. 

External reflectance. As was seen above, in this approach the strong electrolyte absorption is minimised by employing a thin layer configuration and the possible severe restrictions imposed on the diffusion of species to and from the electrode in such an arrangement can lead to some difficulties. 

Before in situ external reflectance FTIR can be employed quantitatively to the study of near-electrode processes, one final experimental problem must be overcome the determination of the thickness of the thin layer between electrode and window. This is a fundamental aspect of the application of this increasingly important technique, marking an obstacle that must be overcome if it is to attain its true potential, due to the dearth of extinction coefficients in the IR available in the literature. In the study of adsorbed species this determination is unimportant, as the extinction coefficients of the absorption bands of the surface species can be determined via coulometry. 

Further evidence came from in situ UV-visible studies by Breikss and Abruna (1986), apparently using external reflectance from a Pt electrode. Figure 3.58(A) shows the absorption spectrum for the rhenium complex in solution in the absence of C02 prior to electrolysis. The potential of the working electrode was then slowly ramped to more cathodic values until it reached —1.7 V, whereupon a reddish material formed with a spectrum labelled B in Figure 3.58, having a Am = 512nm. The potential was then... 

For crystals which have flat faces which extend for a fraction of 1 ym, a new type of phenomenon may be observed. Electrons incident at the edge of the crystal parallel to the surface may be channelled along the surface. The potential field of the crystal extending into the vacuum deflects the electrons so that they tend to enter the surface but they are scattered out of the crystal by the surface atoms or by diffraction from the crystal lattice planes parallel to the surface. If the scattering angle is less than the critical angle for total external reflection, the scattered electrons can not surmount the external potential barrier and are deflected back into the crystal (figure 4 (a)). 

When mixed SAMs of NMB and DMAMB were prepared in toluene, the surface NO2 concentration, as determined by external reflection FTIR spectroscopy, displays a plateau at about 40%. If one assumes that the equilibrium concentration of the two components in the mixed SAM, in a nonpolar solvent, is driven by the formation of a two-dimensional assembly with zero net dipole moment, the results can be explained by using the Hammett equation. 

A majority of traditional NIR measurements are made on solid materials and these involve reflectance measurements, notably via diffuse reflectance. Likewise, in the mid-IR not all spectral measurements involve the transmission of radiation. Such measurements include internal reflectance (also known as attenuated total reflectance, ATR), external reflectance (front surface, mirror -style or specular reflectance), bulk diffuse reflectance (less common in the mid-IR compared to NIR), and photoacoustic determinations. Photoacoustic detection has been applied to trace-level gas measurements and commercial instruments are available based on this mode of detection. It is important to note that the photoacoustic spectrum is a direct measurement of infrared absorption. While most infrared spectra are either directly or indirectly correlated... 

M. Clayboum, External reflection spectroscopy, in Handbook of Vibrational Spectroscopy, l.M. Chalmers and PR. Griffiths (Eds), Sampling Techniques Vol. 2, John WUey Sons, Ltd, Chichester, 2002. 

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