Multiple reflection effects

For thin-film samples, abrupt changes in refractive indices at interfrees give rise to several complicated multiple reflection effects. Baselines become distorted into complex, sinusoidal, fringing patterns, and the intensities of absorption bands can be distorted by multiple reflections of the probe beam. These artifacts are difficult to model realistically and at present are probably the greatest limiters for quantitative work in thin films. Note, however, that these interferences are functions of the complex refractive index, thickness, and morphology of the layers. Thus, properly analyzed, useful information beyond that of chemical bonding potentially may be extracted from the FTIR speara. 

As already mentioned, the reflection modulation technique is relatively simple but for thicker or absorbing thin films it becomes much more complicated due to the multiple reflection effects in the used multilayer structure (Fig. 1) which may lead to erroneous results, if not correctly taken into account [10,20]. In that case, the measurement of the incidence angle dependence of the modulation intensity is required. Through a correct analysis of experimental data one can get both real and imaginary parts of r, as well as its anisotropy and ri3 tensor components). 

FIGURE 6 (a) Graphical construction for multiple diffraction on a square planar reciprocal lattice net.14 (b) Dependence, on rotation angle around die diffraction vector, of multiple reflection contribution to the apparent reflection intensity of 222, a forbidden reflection, in germanium.9 (c) Multiple reflection effects for several unrelated reflections, in rotation around the diffraction vector, showing peaks and dips. 1... 

Snail, K. A. Multiple reflection effects in hemi-ellipsoid reflectometers, presentation at the OSA Annual Meeting, October 20,1987, Rochester, NY,... 

Although the theoretical problems connected with multiple reflection effects have been solved in a general case (Bethune (1988)) in practice one base to take account of experimental conditions like e.g. finite beam diameter, not complete superposition of reflected and incident beam, depending on incidence angle and thin film thickness, negligible reflection in some cases between thin fim and substrate (close refractive indices) etc.. 

The curves in Figure B-1 represent primarily the transient nature of blast waves. They do not represent the interaction effects of blast waves and structures, such as multiple reflections and shielding due to the presence of other structures. 

Figure 12,4.1 The multiple reflection of light from microscopic oxide layers of different dimensions leads to constructive and destructive interference of light waves, producing a particular color effect. Different thicknesses reflect different colors.

The two examples shown demonstrate how an interference effect can be produced either by amplitude division of an incident beam, followed by retardation (achieved in this case by multiple reflection between the partially reflective parallel plates of a Fabry-Perot resonator) and recombination, or by division of the wave front at the multiple equally spaced slits of a diffraction grating, again followed by recombination. 

Multiple reflection of light on a thin solid film with a high refractive index (Fig. 72) causes interference effects in the reflected light and in the complementary transmitted light. For the simple case of nearly perpendicular incidence, the intensity of the reflectance (/) depends on the refractive indices (n1, n2), the layer thickness (d), and the wavelength (A) [5.204], [5.205] ... 

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