Radiation plane polarized

If the electric fields from all the photons that travel in the same direction were forced into the same plane, the radiation is said to be plane polarized or linearly polarized. Radiation will retain the plane of polarization unless it is perturbed in specific ways. As discussed briefly in Section 1.6, plane-polarized radiation can be used to measure the orientation of functional groups in a sample. Oriented samples absorb radiation that is plane polarized in two perpendicular directions to different extents such samples are said to exhibit linear dichroism. Linear dichroism is used to measure orientation in samples in other words, the alignment of functional groups in different physical directions. This topic is discussed in more detail in Section 12.4. 

Polarization is also important in reflection spectrometry, especially for the measurement of thin films on metallic and dielectric surfaces, as described in Chapter 13. For reflection measurements, radiation that is polarized such that its plane of polarization is parallel to the plane containing the incident and reflected rays (the plane of incidence) is said to be p-polarized. Radiation that is polarized such that its plane of polarization is perpendicular to the plane of incidence is said to be j -polarized (from the German word for perpendicular, senkrecht). 

Fourier Transform Infrared Spectrometry, Second Edition, by Peter R. Griffiths and James A. de Haseth Copyright 2007 John Wiley Sons, Inc. 

Polarization does not have to be restricted to a plane. It is possible to cause the electrical field to rotate as the wave propagates. If an observer could record the electric field vector of a wave as it was directed away from the observer, he would see a single line at a fixed angle for plane-polarized radiation. On the other hand, if the electric field vector rotated, the observer would see the angle sweep through 360°. This type of radiation is said to be circularly polarized, and the difference in absorption of radiation that is circularly polarized in opposite directions (clockwise and counterclockwise) by chiral compounds is known as circular dichroism (CD). 

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Let the coordinate system be such as that given in Figure 4. IS. The electric vectors of a plane polarized radiation vibrate along OZ in the ZX plane and OX is the direction of propagation of the plane polarized wave. When a solution of anisotropic molecules is exposed to this plane polarized radiation, the electric vector will find the solute molecules in random orientation. Only those molecules absorb with maximum probability which have their transition moment oriented parallel to OZ (photoselection). Those molecules which are oriented by an angle 6 to this direction will have their absorption probability reduced by a factor cos 6, and the intensity of absorption by cos2 6. Finally, the molecules oriented perpendicular to the electric vector will not absorb at all. These statements are direct consequences of directional nature of light absorption... 

Mathematical Equation for the Combination of Two Plane Polarized Radiation... 

Plane-polarized radiation comprises two circularly polarized vectors of equal intensity, one right-handed and the other left-handed (Fig. B3.5.3A), which are separately measured in the CD spectrometer by means of a photoelastic modulator. A chromophore situated... 

Figure B3.5.3 The relation of ellipticity to the differential absorption of circularly polarized radiation. The oscillating radiation sine wave, 01, is proceeding out of the plane of the paper towards the viewer. (A) Plane-polarized radiation is made up of left- and right-handed circularly polarized components, OL and OR, respectively. Absorption by a chromophore in a nonchiral environment results in an equal reduction in intensity of each component, whose resultant is a vector oscillating only in the vertical plane—i.e., plane-polarized radiation. (B) Interaction of the radiation with achiral chromophore leads to unequal absorption, so that combination of the emerging vectors, OL and OR, leads to a resultant that describes an elliptical path as it progresses out of the plane of the paper. The ratio of the major and minor axes of the ellipse is expressed by tan 0, thus defining ellipticity. The major axis of the ellipse makes an angle (q) with the original plane, which defines the optical rotation. This figure thus demonstrates the close relation between optical rotation and circular dichroism.

Plane-polarized radiation, CD analysis of proteins, 219-243. see also Circular dichroism Plant cell walls fractionation... 

The scattering of X rays by matter consists of two processes. The first process is the classical incoherent scattering, with no change in wavelength, called Thomson scattering, where the average intensity (S) of plane-polarized radiation, due to an electric field E, incident on a single electron is... 

Neither of the two earlier reviews on the infrared spectroscopy of carbohydrates " dealt with the uses of plane-polarized radiation. This is, no doubt, attributable to the fact that both reviews were principally concerned with crystalline sugars, for which few such spectra are available. However, the polarized infrared spectra of such polysaccharides as cellulose, chitin, and xylans, in the form of oriented films, have been measured " and have provided information that other techniques could not give. It is, therefore, desirable that a brief discussion should be here provided of both the experimental and the interpretational aspects (see p. 28) for a detailed discussion, the reader is referred elsewhere. The commonest method of obtaining polarized infrared radiation is with a transmission polarizer (rather than a reflection polarizer). Selenium film and silver chloride sheet have both been used of these, the latter is the more popular because it is the more robust. A stack of about six sheets, each about 50-100 m thick, is... 

The same orbital sequence had earlier been established for square PtCL , using the same technique of measuring the spectra of oriented single crystals with plane-polarized radiation. The selection rules... 

Optical activity of natural products may depend on chemical factors such as asymmetric carbon atoms, restricted rotation, etc. These may be termed primary structural features. There are also secondary structures, e.g., helices or random coils, that may confer chirality to a natural product. Optical rotatory dispersion (ORD, i.e., rotation of plane-polarized radiation over a range of wave-lengths usually from approximately 200 to approximately 500/im) has been used in studies of the conformations of many different molecules, including polymers, proteins, and polypeptides [90]. 

A scattering process will also result in polarization of x-rays according to the classical Thompson theory expressed by Eq. (2.1). For an unpolarized primary beam, scattering at an angle (26) of 90° results in nearly complete plane polarization of the scattered x-rays. Thus, crystal diffraction of x-rays produces plane-polarized radiation. If the diffraction angle (26) approaches 90°, polarization becomes nearly complete. 

The cos a factor causes the differential cross-section to be small for small values of a so that plane-polarized radiation is preferentially scattered out of the plane of polarization ... 

For plane-polarized radiation, the differential cross-section is given by... 

If plane-polarized radiation enters a transparent isotropic body parallel to a magnetic field, the plane of polarization is rotated (magnetooptical rotation, MOR, or Faraday effect) (40). The angle... 

Yet, the optical activity must be carefully differentiated by the birefringence, but can accompany it as a perturbation . Thus, the optical activity can be investigated based on the birefringence, when one of the monochromatic plane-polarized radiations continue to be used for a new incidence on a crystal, and recording, in transmission, the rotation of the polarization plane respecting the incident polarization direction. 

The radiation produced by the coil of an RF oscillator, which serves as the source in NMR instruments, is plane polarized. Plane-polarized radiation, however, consists of d and / circularly polarized radiation. As shown in Figure 19-4b, the vector of the d component rotates clockwise as the radiation approaches the observer the vector of the / component rotates in the opposite sense. Addition of the two vectors leads to a vector sum that vibrates in a single plane (Figure l9-4a). 

FIGURE 6-11 Unpolarized and plane-polarized radiation (a) cross-sectional view of a beam of monochromatic radiation, (b) successive end-on view of the radiation in (a) if it is unpolarized, (c) successive end-on views of the radiation of (a) if it is plane polarized on the vertical axis. 

In the infrared, plane polarized radiation is required, and is provided by passing the radiation through a polarizer transparent in the wavelength domain of interest. 

For right-circularly polarized radiation the electric field vector rotates clockwise when looking into the oncoming wave, i.e. at the source of the radiation. Circular polarization of photons corresponds to the two possible projections of the photon s spin on the direction of propagation, S, called helicity. Right-circularly polarized photons have = — 1 and thus Sz = —h, while left-circularly polarized photons have nis = 1. Plane-polarized radiation can then be expressed as a superposition of left-and right-circulary polarized waves with the same refractive index, rir uj) = =... 

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