External reflection, spectroscopy

With p-polarized light the phase shift varies from 0 to 180° depending on the actual value of 0. Vectorial addition of the electric field vectors of the impinging and reflected rays yields the effective electric field at the point of reflection. In the case of s-polarized light, the phase shift of approx. 180° will result in practical zero field strength at all values of 0. At a 90° phase shift the effective field will be 

At angles close to 90° the phase shift is at the value yielding a maximum effective field strength. Calculation of absorption coefficients Ap for p-polarized and As for s-polarized light based on the Maxwell equations, assuming a layer of 1 nm of acetone on a reflecting surface, show basically the close relationship between angle of incidence and phase shift (see Fig. 5.46). 

Utilization of the differences in interaction between infrared light of the two possible types of polarization with species at the interface (or in the interphase) allows the dedicated design of setups for measurements of surface species or of solution phase species. The latter possibility has already been discussed and will be considered again briefly at the end of this section (p. 90) the former case will be treated below. 

Surface enhanced infrared absorption (SEIRA) has been observed in external reflection spectroscopy [185], for further details, see Sect. 5.2.5. 

Taking into account the general considerations outlined above the spectroelec-trochemical cell has to be of a thin layer design. A typical example is shown in a cross section in Fig. 5.48 another view is presented in Fig. 5.49, showing major features of the cell. In cases where a standard spectrometer is used, the cell has to be mounted in the optical beam using an external reflection attachment as schematically depicted in Fig. 5.47. 

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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. 

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. 

FTIR-ERS Fourier Transform Infrared external reflection spectroscopy... 

When the surfaces are highly reflecting as in the case of metals, external reflection spectroscopy (ERS) can be used with good success133). For optimum intensity of the reflection bands of thin films, angles of incidence near 88 are desirable. However, in order not to interfere with the incoming beam, angles of incidence near 80° are used. 

Claybourn, M., External Reflection Spectroscopy. In Chalmers, J.M. 8c Griffiths, P.R. (eds) Handbook of Vibrational Spectroscopy, Sampling Techniques, Volume 2 John Wiley 8t Sons Chichester, 2002 pp. 969-981. 

Reflectance. Both internal and external reflectance spectroscopy are relatively simple experiments to perform. Commercially available attachments for standard UV-visible spectrometers can be used. For films with strong electronic transitions reasonable spectra can be obtained. The theory for external and internal reflectance is the same as that for the IR and can be found elsewhere (2, 37). The techniques have not been very popular in their applications to surface analysis. The major reason appears to be... 

Fourier transform infrared microscopes are equipped with a reflection capability that can be used under these circumstances. External reflection spectroscopy (ERS) requires a flat, reflective surface, and the results are sensitive to the polarization of the incident beam as well as the angle of incidence. Additionally, the orientations of the electric dipoles in the films are important to the selection rules and the intensities of the reflected beam. In reflectance measurements, the spectra are a function of the dispersion in the refractive index and the spectra obtained are completely different from that obtained through a transmission measurement that is strongly influenced by the absorption index, k. However, a complex refractive index, n + ik can be determined through a well-known mathematical route, namely, the Kramers-Kronig analysis. 

Teresa Iwasita and F. C. Nart provide a valuable perspective on the foundations, capabilities, and limitations of in-situ infrared external reflection spectroscopy of electrode surfaces, with emphasis on Fourier Transform instruments. In addition to the description of underlying principles and instrumentation, selected examples are given of the monitoring and interpretation of spectra of various species adsorbed at electrochemical interfaces. 

A consistent description of the structure of alkanethiol monolayers on gold has emerged from an array of spectroscopic and diffraction studies. X-ray photoelectron spectroscopic (XPS) studies support the presence of anisotropically chemisorbed alkanethiolates on gold [24-29]. Ellipsometric measurements [24-27, 30], capacitance studies [30] and XPS measurements [31] confirm monolayer film thickness. Fourier transform infrared external reflective spectroscopy (FTIR-ERS) shows that the chains tilt at about 30° off the surface normal, and the plane containing the carbon backbone is twisted out of the plane of tilt by about 50° [25-27, 30, 32, 33]. 

Besides cell designs that employ metals or other electrode materials in disc shape for external reflection spectroscopy, Robinson and McCreery have successfully employed cylindrical carbon fibers of 12 pm diameter [58, 59]. The carbon fiber was illuminated with a tunable dye laser. Scattered light was collected with fiber optics and guided to a photomultiplier detector. Because no thin layer arrangement and consequently poor electrochemical cell response were involved, fast experiments on a microsecond time scale were possible. Studies of polyaniline films deposited on platinum discs have been described [60]. 

A combination of transmission and external reflectance spectroscopy resulting in a cell for bidimensional UV-Vis spectroelectrochemistry has been described [61]. With an optically transparent electrode (OTL), the schematic setup shown in Fig. 5.8 illustrates the different pathways of the light. One beam passes through the electrode and the electrolyte solution in front of it and the second beam passes only through the solution in front of the electrode close to it, guided strictly in parallel to the surface. Thus the former beam carries information pertaining to both the solution and the electrochemical interface (e.g. polymer films or other modifications on the electrode surface), whereas the latter beam carries only information about the solution phase. Proper data treatment enables separation of both parts. Identification of... 

Claybom M, Umemura J, Merklin GT, Kattner J, Hoffinann H, Mendelsohn R, Flach CR, Frey BL, Com RM, Weibel SC, Rdseler A, Korte E-H (2002) Mid-infrared external reflection spectroscopy. In Griffiths PR, Chahners J (eds) Handbook of vibrational spectroscopy, vol 2. Wiley, Chichester, pp 969-1090... 

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