Polarized Radiation Studies

In such instances no knowledge of the orientation distribution or of / is required to compute 6V Once 6X is obtained, / can be calculated from equation (49), and using this / the 6 values for other moments can be obtained directly from the observed dichroic ratios. These relations demonstrate the potentialities of polarized radiation studies in providing structural information on high polymers. 

The infrared active v (CH2), v (CH2), 8 (CH2), and yr (CH2) fundamentals can be readily assigned as a result of the extensive spectroscopic studies on hydrocarbons which have been undertaken [Sheppard and Simpson (795)]. In addition, because of the polarized radiation studies on single crystals of normal paraffins [Krimm (95)], it is possible to assign uniquely the components of the doublets found in the spectrum for these bands to symmetry species. Similarly, the Raman active va(CH. ), vs(CH2), (CHg), v+ (0), and v+ (n) fundamentals can be unambiguously assigned, the latter two on the basis of normal vibration calculations... 

It may appear from the above that only nonpolar liquids yield nonmolecular solvent anions upon the ionization. Perhaps this is misleading Most polar liquids studied by radiation chemists are aliphatic alcohols and water, and these liquids yield solvated electrons rather than radical ions. Although there has been sporadic interest in other polar liquids (e.g., neat acetone), the current state of knowledge of such systems does not allow one to reach any conclusion as to the nature of the reducing species observed therein (although, see Sec. 4). 

Unlike grazing angle reflection studies of films on metals, both parallel and perpendicularly polarized radiation is observable upon reflection from the water surface. Similar to ATR experiments, electric fields in all three dimensions relative to the water surface are present, indicating the potential for complete orientation studies of the surface layers 126). 

Figures 4A and 4B present experimental spectra that illustrate the principle that the incoming E field distribution helps govern the type of spectra obtained. In Figure 4A, spectra of a DPPC monolayer are presented which were obtained at 60 angle of incidence with s-polarized radiation. As in previous studies where the experimental angle of incidence was 30° (2-6), the observed spectra have negative absorbances. In Figure 4B, however, the spectra of the monolayer taken with p-polarized radiation show positive absorbance bands, as predicted from theory (Figure 3).

When spectroscopy with polarized light or other optical polarization measurements are performed the studied sample becomes one element of the optical train which transmits the beam from the radiation source to the detector. Four optical phenomena might take place in a sample sensitive to polarized radiation. We list them below together with their physical definition ... 

The matter has been carried further in the studies of Gratzer et al. (1961) with measurements of the absorption of polarized radiation by oriented films of a-helical polypeptides. Their polarization spectra of poly-L-ala-nine, and of poly-7-methyl-L-glutamate (Fig. 8) clearly show the predicted opposite polarization for the two peptide absorption bands of the -helix. Taking these band positions as 1910 and 2060 A, we obtain a band separation of 3800 cm . [The value of 2700 cm in the paper of Gratzer et al. 

The adsorption of cyanide on Pd electrodes was studied by using a combination of polarization and potential modulation (FT-IRRAS and SNIFTIRS) [124]. The reason for this combination is to enhance the surface signal, since two FT-IRRAS spectra taken at two different potentials were ratioed to obtain the SNIFTIRS spectrum. Despite this effort, a solution band at 2135 cm" (also observed with s-polar-ized light) persists in the spectrum (Fig. 36). The spectrum in this Figure taken with p-polarized radiation presents, however, a strong band at 1980 cm", which was attributed to a bridged-bonded cyanide ion, and a weak band at 2065 cm" assigned to linearly adsorbed C-down CN". 

The preferred excitation-detection geometry for the study of macroscopic samples is the right angle geometry [24]. In order to maximize the polarization of the emitted fluorescence, excitation is made with linearly polarized radiation. Tire direction of polarization is usually the vertical direction (see Fig. 7.11). 

Another example of the use of polarized radiation in imaging studies is the analysis of poly(vinylidene fluoride)(PVDF) films, which have been uniaxially elongated at different temperatures. Depending on the thermal, mechanical and electrical pretreatment, PVDF can exist in different modifications [59]. The crystal structure of the cmmpled 11(a) modification can be converted into the aU-tra s 1(P) form by tensile stress below 140°C (see Figure 9.27a). Figure 9.27b shows the stress-strain diagrams of PVDF films in the 11(a) form which have been elongated to 400 % strain at 100 and 150°C. The observed decrease in stress upon elevation of the... 

From expression (37), it is possible to study the effect of beam polarization on the interaction energy. This may be carried out for linearly and circularly polarized radiation propagating in directions parallel and perpendicular to the intermolecular join, R. Details and results may be found in Refs. [18,19]. 

Much attention has been given to the analysis of OH stretching bands of micas occurring in an isolated region of the spectrum ranging from about 3750 to 3550 cm (Farmer 1974). Using polarized radiation, the pleochroic scheme of IR absorption bands, measured on oriented crystal sections, makes it possible to impose defined constraints on the orientation of the OH dipoles. In a fundamental polarised IR study, Tsuboi (1950) determined the position of the H atom in the structure of muscovite. The variation of absorption intensity of the OH stretching band with the direction of the electric vector of... 

On a submicro scale, techniques of the kind developed by Caspersson (1936 and 1940) and his school, as also those of Cole and Brackett (1940) and Thorell (1947), are essentially extensions and developments of macro-spectrophotometric methods. Criticisms of such techniques must arise from doubts as to the validity of Beer-Lambert laws of absorption in small inhomogeneous objects in uncollimated beams (Commoner, 1949, Commoner and Lipkin, 1949). However these techniques when applied to certain selected objects, e.g. crystals, are free from this criticism of inhomogeneity of material and may be used in much the same manner as the macrotechniques, with the added advantage that the sample may be solid and the effect of molecular orientation studied in polarized radiation. 

Numerical experiments of Ivanov and Kasaurov [12], who studied the behavior of iterations for the transformed integral equation for the source function in the case of scalar anisotropic scattering, revealed that with an appropriate choice of parameters, a dramatic acceleration of convergence can always be achieved. One should expect that the same also is trae for iterative computational methods based on the -transformed transfer equation for polarized radiation derived above. 

In this report we demonstrate the similarity of the quadratic integrals for unpolarized radiation, as employed by Rybicki, to the quadratic integrals used by Siewert and McCormick for the 2-vector polarized radiation problem. We have assumed that both I- and Q-components may be measured to the same accuracy. This may not be the case, as pointed out by Mishchenko, [10] because the Q-component of the unpolarized incident flux is much smaller than /-component. How the difference in measurement errors influences the results remains to be studied in a forthcoming paper. 

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]. 

Higuchi et al. (1969) have used polarized radiation to study the spectrum (Fig. 12.7) of an oriented film of a DNA-RNA hybrid in its undeuterated and deuterated forms. The band at 1225 cm displayed perpendicular dichroism and the one at... 

Electronic Absorption Spectra (58, 59). As seen earlier, solvent effects on reactivity and electronic absorption have the same origin. Historically, however, the former were studied in terms of classical electrostatics, whereas the latter generally underwent (at least, formally) a quantum-mechanical treatment. Liptay (58/) has shown that the results of both theories are the same (at the level of the second order perturbation theory) except for a term corresponding to dispersion interactions. Assuming that the polar, radiation-absorbing species is in a very dilute solution, the main contributors to the energy of the... 

Circular dichroism (CD) spectroscopy is based on the difference of absorption of right and left circularly polarized radiation by chiral (dissymmetric) molecules. In peptide research CD spectra can give valuable information on the conformation of peptide molecules in solution. The differences in absorption (ellipticities, 9) are pronounced in the 200 to 220 nm region. The positive or negative Cotton effects allow empirical statements on the arrangement of peptide chains, through comparisons with the CD spectra of peptides that have helical, pleated sheet or random coil conformations, established by other means, e.g. by X-ray diffraction studies. Optical rotatory dispersion (ORD) spectroscopy, which is based on the different refraction of circularly polarized light by chiral compounds, is the older... 

---