Optical Anisotropy Correction

Another method for correcting for optical anisotropy has been described by Stacey (1956) and applied to lignosulfonates (Forss and Stenlund 1969). It involves essentially the multiplication of the measured Kc/AR(0) values by the Cabannes factor, expressed for the case in which the incident light is vertically polarized (Armizadeh and McDonnel 1982), as  

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As mentioned above, with lignins, optical anisotropy, absorbance, and fluorescence must be corrected for or minimized (Pla et al. 1977). 

Hence, to understand correctly the mechanism of flow birefringence of r d-chain polymers it is necessary to take into account both the asymmetry of the pe of their molecules and their optical anisotropy. Both the properties are the main properties of the molecule and if one of them is absent, flow birefringence in solution of kinetically rigid molecules is impossible. 

Since attention is now focussed on the amorphous portion of the gel, X-ray analysis can no longer serve to obtain a quite correct measure of orientation and the latter should be derived from measurements of optical anisotropy. We have already seen on p. 638 that in the process of orientation by stretching, the optical orientation factor lags behind that derived from X-ray analysis. On page 593 it has been explained that the optical orientation factor is a measure of the average orientation of both the crystalline and amorphous components. However, since the latter forms the major portion of the gel, it will mainly determine the optical orientation factor. In some favourable cases the contribution of the crystalline component can, moreover, be computed and corrected for. 

Anisotropy in polymer solutions is rather rare, but, where it occurs, the effect on the derived molecular weight can be large enough to warrant appropriate corrections. The procedures have been developed by Utiyama124) and by Utiyama and Kurata125). Without a polariser or analyser the normal reciprocal scattering function Kc/Rtf can be measured. It is denoted by Z(0) the form of which contains an anisotropy parameter 5 which is a function of the number of optically anisotropic elements and the principal polarisabilities in the chain ... 

When the incident light is horizontally polarized, the horizontal Ox axis is an axis of symmetry for the fluorescence intensity Iy = Iz. The fluorescence observed in the direction of this axis (i.e. at 90° in a horizontal plane) should thus be unpolarized (Figure 5.3). This configuration is of practical interest in checking the possible residual polarization due to imperfect optical tuning. When a monochromator is used for observation, the polarization observed is due to the dependence of its transmission efficiency on the polarization of light. Then, measurement of the polarization with a horizontally polarized incident beam permits correction to get the true emission anisotropy (see Section 6.1.6). 

The anisotropy, that is, the maxi mum minimum scattering intensity ratio /max//min, seems to follow the energy-transfer spectrum as illustrated by Fig. 35. Taking account of some suitable corrections for the angular spread of the initial CM direction and for incomplete optical pumping, the maximum anisotropy is determined to be 26% for N2 at 80°K, 0lab= 10°, and CM 1.2 eV-... 

In practice, the optics and the detection system respond differently to different polarizations, and a correction factor G must be introduced in Eq. (7.16) for the calculation of anisotropy [16a] ... 

The polarization characteristics of monochromators have important consequences in the measurement of fluorescence anisotropy. Such measurements must be corrected for the varying efficiencies of each optical component this correction is expressed as the G-factor (Section 10.4). However, the extreme properties of the concave gratings (Figure 2.11) can cause difficulties in the measurement of fluorescence polarization. For example, assume that the polarization is to be measured at an excitation wavelength of 450 nm. The excitation intensities will be nearly equal with the excitation polarizers in each orientation, which makes it easier to compare the relative emission intensities. If the nission is unpdarized, the relative intensities of the parallel (U) and perpendicular (X) excit ion will be nearly equal. However, suppose the excic ion is at 340 nm, in which case the intensities the... 

All the Raman data previously reported did not take into account the true values of the optical function anisotropy of these materials. This is due to intrinsic difficulties in measuring the fundamental optical constants of these samples. In fact, it is expected that large corrections, due to reflectivity and absorption, should be applied to the observed Raman intensities. Simple models have been proposed [1,3] to roughly estimate these effects, using approximated values of the optical functions, deduced from theoretical models [2] or from measurements on similai samples [1,3]. 

Raw observed data should be corrected using a reference spectrum obtained in the same experimental condition in order to remove artifacts such as anisotropy of the optical cell and intrinsic nonzero baseline of the instrument. A general way to correct a raw CD spectrum of a solution-state sample is solvent correction. 

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