Reflection ellipsometry techniques

The optical constant such as refractive index, and the thickness of a film are determined by ellipsometry. For polymer thin films the reflection ellipsometry technique is commonly used. This technique is especially useful in the wavelength regions where the materials are strongly absorbing so that the transmission measurements are precluded. 

Iv) Ellipsometry. With ellipsometry one measures not only reflected intensities, but also the changes in polarization upon reflection. The technique has been discussed in sec. 1.7.10b. 

The principle of the reflectance spectroscopy technique is to shine a monochromatic light beam, usually polarised in a plane parallel or perpendicular to the plane of incidence, at an electrode surface at a known angle of incidence and to record the intensity of the reflected beam as a function of either wavelength, potential or time. The data obtained are then compared to model calculations made using the same three layer model as is used in ellipsometry, i.e. bulk electrode/interfacial region/bulk solution. These calculations are based on the Fresnel equations [19] and assume that all phases are isotropic. 

The progress in this area is, to a significant degree, linked to the exploitation of specialized experimental techniques such as neutron reflection, ellipsometry, fluorescence... 

This chapter summarizes the optical principles involved in ellipsometry and reviews some typical applications in electrochemical systems. Newly developing areas of application and recent developments in the experimental approach and instrumentation will also be dealt with. Some emphasis will be on modified techniques, including the combined reflectance-ellipsometry method. Ellipsometry is a broad field that includes various techniques and a wide range of applications. The chapter is mostly devoted to showing what can be done with ellipsometry for the purpose of investigating electrochemical interfaces. Readers are referred to other sources of information on specific subjects. A thorough treatment of polarized light and ellipsometry has been published by Azzam and Bashara. The technique as applied to electrochemistry has also been subject to various reviews " and symposia. 

The optical principles and equations used in the technique are concisely summarized. The combined reflectance-ellipsometry (three-parameter ellipsometry) method and spectroscopic ellip-sometry are expected to be applied to an increasing number of studies in interfacial electrochemistry. The importance of proper experimental conditions, especially the proper choice of incidence angle is emphasized. Instrumentation, experimental methods, and error and sensitivity problems are dealt with. Some typical and recent applications in electrochemistry are reviewed. 

A quite different means for the experimental determination of surface excess quantities is ellipsometry. The technique is discussed in Section IV-3D, and it is sufficient to note here that the method allows the calculation of the thickness of an adsorbed film from the ellipticity produced in light reflected from the film covered surface. If this thickness, t, is known, F may be calculated from the relationship F = t/V, where V is the molecular volume. This last may be estimated either from molecular models or from the bulk liquid density. 

The detailed examination of the behavior of light passing through or reflected by an interface can, in principle, allow the determination of the monolayer thickness, its index of refiraction and absorption coefficient as a function of wavelength. The subjects of ellipsometry, spectroscopy, and x-ray reflection deal with this goal we sketch these techniques here. 

Whereas the XSW technique takes advantage of the standing wave established on the total reflection of X-rays from a mirror surface, a conceptually more straightforward approach is that of simply specularly reflecting an X-ray beam from an electrode coated with the film of interest, measuring the ratio of the intensities of the incident and reflected rays, and fitting the data, using the Fresnel equations, to a suitable model an approach similar to optical ellipsometry. 

Until quite recently the very initial stages of metal deposition were difficult to characterize in detail by structure- and morphology-sensitive techniques. As a consequence and for practical purposes - multilayers were more useful for applications than monolayers - the main interest was focussed onto thick deposits. Optical and electron microscopy, ellipsometry and specular or diffuse reflectance spectroscopy were the classic tools, by which the emerging shape of the deposit was monitored [4-7],... 

Unlike the dependence of Aeff on film thickness alone (dNc /dd) that is sometimes used as a figure of merit for guided mode molecular sensors, 5m0d captures both the index and thickness dependence of the sensor response in a single parameter. While Dopt does not uniquely determine the film response for other optical techniques such as ellipsometry and reflectance difference, once d and n of the film are known, the optical thickness can be evaluated and comparisons are made between guided mode sensors and other techniques. 

The heart of the polarization-modulated nephelometer is a photoelastic modulator, developed by Kemp (1969) and by Jasperson and Schnatterly (1969). The latter used their instrument for ellipsometry of light reflected by solid surfaces (the application described here could be considered as ellipsometry of scattered light). Kemp first used the modulation technique in laboratory studies but soon found a fertile field of application in astrophysics the modulator, coupled with a telescope, allowed circular polarization from astronomical objects to be detected at much lower levels than previously possible. 

Reference electrode, 1104, 1108, 1113 potential, 819, 874 Refractive index, determination with ellipsometry, 1148. 1151 Reflection coefficient, 1151 Residence time, definition, 1310 Reversal techniques, determination of intermediate radicals, 1416 Reversible adsorption of organic molecules, 969, 970... 

With all its complications and uncertainties, impedance spectroscopy, as seen at the end of the twentieth century, is a growing technique in fundamental electrodic analysis [cf. the seminal contributions of (independently) D. D. and J. R. MacDonald]. Among its advantages is that the necessary equipment is less expensive than that of competing spectroscopic equipment and that it can provide information on any electrochemical situation (e.g., it is not limited by, say, the need for specular reflectance, as in ellipsometry). 

The thicknesses of free soap films and liquid films adsorbed on surfaces (Figs. 1.26d and 1.26e), which can be measured using optical techniques such as reflected intensity, total internal reflection spectroscopy, or ellipsometry as functions of salt concentration or vapor pressure, can provide information on the long-range repulsive forces stabilizing thick wetting films. We see an example of this in Chapter 11. 

McCrackin, F. L., Colson, J. P. Computational techniques for the use of the exact drude equations in reflection problems, in Ellipsometry in the Measurement of Surfaces and Thin Films (eds.) Passaglia, E., Stromberg, R. R., Kurger, J., p. 61, NBS Miscellaneous Publication 256, Washington, D. C., Superintendent of Documents, U. S. Government Printing Office 1964... 

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