Refraction, radiation scattering

In the second broad class of spectroscopy, the electromagnetic radiation undergoes a change in amplitude, phase angle, polarization, or direction of propagation as a result of its refraction, reflection, scattering, diffraction, or dispersion by the sample. Several representative spectroscopic techniques are listed in Table 10.2. 

Adsorption of radiation Scattering of radiation Refraction of radiation Diffraction of radiation Rotation of radiation Electrical potential Electrical current Electrical resistance Mass to charge ratio Rate of reaction Thermal properties Mass Volume... 

Figure 2 The scattering problem illustrated for a spheroidal particle with orientation e and effective refractive index = n — i k. The surrounding medium is nonabsorbing, with the real refractive index ne, The speed of light within the medium is c = Co/r e, where Cq is the speed of light in vacuum. The incident plane wave has frequency v (ie, wavelength X = dv) and a wave vector collinear to o. A propagation direction of the radiation scattered by the particle is denoted as a. 0 is the angle between a and co. ...

Insulating or weakly semiconducting materials, like most inorganic compounds, do not absorb the radiation outside the skeletal region, where all light is essentially reflected, transmitted, refracted, or scattered. The skeletal vibrations give rise not only to absorbed radiation in transmission and diffuse reflectance experiments, but also to reflected radiation in the reflection experiments. Thus the specular reflectance for insulating materials, both in the form of monocrystals and sintered pellets, is frequently the basis for the best determination of the skeletal spectrum, as far as the IR-active modes are concerned. 

In Raman spectroscopy the intensity of scattered radiation depends not only on the polarizability and concentration of the analyte molecules, but also on the optical properties of the sample and the adjustment of the instrument. Absolute Raman intensities are not, therefore, inherently a very accurate measure of concentration. These intensities are, of course, useful for quantification under well-defined experimental conditions and for well characterized samples otherwise relative intensities should be used instead. Raman bands of the major component, the solvent, or another component of known concentration can be used as internal standards. For isotropic phases, intensity ratios of Raman bands of the analyte and the reference compound depend linearly on the concentration ratio over a wide concentration range and are, therefore, very well-suited for quantification. Changes of temperature and the refractive index of the sample can, however, influence Raman intensities, and the band positions can be shifted by different solvation at higher concentrations or... 

The signal generation principle of Raman is inelastic molecular light scattering, in contrast to resonant energy absorption/emission in IR spectroscopy. During the measurement, the sample is irradiated with intense monochromatic radiation. While most of this radiation is transmitted, refracted or reflected, a small amount is scattered at the molecules. 

The detection limit of the microresonator-based refractive index sensing device is directly related to the g-factor of the resonator and the sensitivity of the resonant mode discussed above. The g-factor of a microtube resonator is determined by the total loss of a resonant mode, including radiation loss, absorption loss, and surface roughness scattering loss. The overall g-factor can be expressed as... 

A quantitative description does become possible, however, if the system under examination satisfies special conditions. These include diffuse, monochromatic illumination, homogeneous pigmentation, isotropic scattering in the coating, no difference in refractive index between vehicle and air, and a coating so thick that the substrate has no effect on the exiting radiation. This is the special case treated by the Kubelka-Munk theory. 

Work on the structure of crystals and fibers was not the only way in which Mark made use of x-rays. With several collaborators, he reported the results of a number of significant investigations of the physics of x-rays in 1926 and 1927. With Ehrenberg he reported studies of the index of refraction of x-rays, and with Leo Szilard studies verifying the linear polarization of x-rays scattered from electrons at 90. An investigation of the width of x-ray lines was carried out by Mark and Ehrenberg, and Mark and Kallmann reported work on the properties of Compton-scattered x-radiation and on the theory of the dispersion and scattering of x-rays. 

In diffuse reflection spectroscopy, the spectrometer beam is reflected from, scattered by, or transmitted through the sample, whereas the diffusely scattered light is reflected back and directed to the detector. The other part of the electromagnetic radiation is absorbed or scattered by the sample [124,125]. Changes in band shapes or intensity as well as signal shifts can be affected by morphological and physicochemical properties of the sample or combinations thereof (e.g., chemical absorptions, particle size, refractive index, surface area, crystallinity, porosity, pore size, hardness, and packing density [126]). Therefore, NIR diffuse reflection spectra can be interpreted in dependence of various physical parameters [127]. 

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