Multiple reflectance-absorption

A modification of the simple grazing-angle experiment is the multiple reflection -absorption spectroscopy, which was first described by Gaines (Figure 6) (24). It has a unique value for the detection of order-disorder transformations in the structure of a... 

Other systematic errors include absorption, extinction, and multiple reflection. Absorption effects and the required corrections are well understood. When a monochromatic beam of X rays or neutrons with incident intensity / passes through a crystal, the intensity is reduced exponentially... 

Multiple Reflection Absorption Infrared Spectroscopy (MRAIRS) since these techniques offer complementary information." (See Infrared spectroscopy). 

It is interesting to note that a similar specttum of the 0-0 band of the a-X system, leading to the same value of the absorption intensity, has been obtained using a Fourier transform spectrometer (see Section 3.3.3.2) but with an absorption path, using a multiple reflection cell, of 129 m and half the pressure of gas. 

In the concepts developed above, we have used the kinematic approximation, which is valid for weak diffraction intensities arising from imperfect crystals. For perfect crystals (available thanks to the semiconductor industry), the diffraction intensities are large, and this approximation becomes inadequate. Thus, the dynamical theory must be used. In perfect crystals the incident X rays undergo multiple reflections from atomic planes and the dynamical theory accounts for the interference between these reflections. The attenuation in the crystal is no longer given by absorption (e.g., p) but is determined by the way in which the multiple reflections interfere. When the diffraction conditions are satisfied, the diffracted intensity ft-om perfect crystals is essentially the same as the incident intensity. The diffraction peak widths depend on 26 m and Fjjj and are extremely small (less than... 

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. 

Gaseous samples require long path length cells to produce absorption bands of reasonable intensity up to several metres of optical path are obtainable from cells incorporating mirrors which produce multiple reflections. For GC-IR, light pipes provide the best sensitivity (p. 117). 

Absorption of infrared radiation by characteristic vibrations of a surface can be used to obtain information about that surface, by comparison with known absorption frequencies in molecules of known structure. Surface sensitivity is obtained by using small particles ) and thin films or, better, a multiple-reflection arrangement with optimized angles of incidence and reflection in particular making work on single-... 

This situation is obviously useless for spectroscopy. Hence, valuable films are very thin, on the order of few nanometers, depending on the material. Figure 4 shows that even for a thin metal film the overall reflectivity of the system can be severely reduced. This characteristic is even more important for multiple reflection elements, for which the total reflectivity Rtot is the reflectivity for a single reflection R to the power of the number of reflections N. Rtot = R - For a reflectivity R of 90% (90% of the incident intensity is reflected), only 35% of the incident intensity will reach the detector after 10 reflections (N — 10). The smaller the reflectivity Rtot> the smaller is the signal at the detector and the larger the noise. On the other hand, the intensity of an absorption... 

Nacreous and Interference Pigments. The optical principles of nacreous (interference) pigments are shown in Figure 72 for a simplified case of nearly normal incidence without multiple reflection and absorption. At the interface P, between two materials with refractive indices n t and n2, part of the beam of light L, is reflected (L,) and partially transmitted (i.e., refracted) (L2). The intensity ratios depend on n, and n2. In a multilayer arrangement, as found in pearl or pearlescent and iridescent materials (Fig. 71 C), each interface produces partial reflection. After penetration through several layers, depending on the size and difference between n1 and n2,... 

Diffuse Reflectance, Attenuated Total Reflection or Multiple Internal Reflection, Photoacoustic (PAS), Photothemal Beam Deflection, Specular Reflection Absorption, and forensic applications with the diamond cell and the Fourier transform infrared (FTIR) microscope. In museum laboratories, FTIR applications have been used for problems of identification and degradation in art and archeology. (14)... 

Depending on the absorptivity of the sample, multiple reflection elements (Figure 7) may be necessary, rather them the single reflection as described here. 

Disregarding multiple reflection, the absorptivity, defined as the photon current absorbed in a body of thickness d divided by the incident photon current is... 

Pande et al. 2009 (44) MWCNT CVD As-synthesized Solvent casting CNT Loading level 10 vol.% Two- step method loading levels CNT 10 vol.% Film Bulk Composites EMI SE in X-Band Layered composite (7 layers of 0.3 mm thickness film) - 40 dB Bulk composite (2 layers of 1.1 mm thickness bulk specimen) 30 dB Contribution of SEA and SEK to SEThuiI was studied SE dominated by absorption Stacking method results in increased multiple reflections and absorptions for layered composite ... 

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