Light, electric vector

The classical scheme for dichroism measurements implies measuring absorbances (optical densities) for light electric vector parallel and perpendicular to the orientation of director of a planarly oriented nematic or smectic sample. This approach requires high quality polarizers and planarly oriented samples. The alternative technique [50, 53] utilizes a comparison of the absorbance in the isotropic phase (Dj) with that of a homeotropically oriented smectic phase (Dh). In this case, the apparent order parameter for each vibrational oscillator of interest S (related to a certain molecular fragment) may be calculated as S = l-(Dh/Di) (l/f), where / is the thermal correction factor. The angles of orientation of vibrational oscillators (0) with respect to the normal to the smectic layers may be determined according to the equation... 

Convenient choices for the incident light electric vectors are AQ = (Eq/JI) (1,01 and SQ = (IQ, 0,0,0), which represent circularly polarized... 

The E/Z-isomerization process is characterized by angular-dependent excitation and leads, therefore, to the photoselection of a preferred azobezene dye orientation. In other words, the dichroic dye units choose an orientation where the electronic transition moment is perpendicular to the light electric vector. It promotes, in turn, the cooperative reorientation of neighboring moieties, which include other fragments of the macromolecule, such as the main chain or photochemically inactive comonomer units, and low molar mass additives. Thus, a macroscopic orientation of the sample arises, and it remains long after the illumination is stopped and all the dye moieties return to the thermodynamically equilibratory -state. 

There have also been reports on the preparation of polar materials by a photo-electro-poling technique that combines the optically induced quadrupolar depletion of chromophores in the direction of the light electric vector with an additional field-induced orientation of dipolar chro-mophores. The latter allows the preparation of cold electrets, which are interesting for nonlinear optical applications, such as optical harmonic generation, wave mixing, etc. ... 

If the polymeric molecules are perfectly aligned with the fiber axis, the ratio (Ro) of the integrated intensities measured with the plane-polarized light (electric vector vibrating parallel and perpendicular to the fiber axis) and the inclination (a) of the transition moment direction to the fiber axis is related by... 

On metals in particular, the dependence of the radiation absorption by surface species on the orientation of the electrical vector can be fiilly exploited by using one of the several polarization techniques developed over the past few decades [27, 28, 29 and 30], The idea behind all those approaches is to acquire the p-to-s polarized light intensity ratio during each single IR interferometer scan since the adsorbate only absorbs the p-polarized component, that spectral ratio provides absorbance infonnation for the surface species exclusively. Polarization-modulation mediods provide the added advantage of being able to discriminate between the signals due to adsorbates and those from gas or liquid molecules. Thanks to this, RAIRS data on species chemisorbed on metals have been successfidly acquired in situ under catalytic conditions [31], and even in electrochemical cells [32]. 

FIGURE 27.23 Electric (E) and magnetic (H) vectors in a linearly polarized light wave. The plane of polarization contains the electric field vectors in space. At a fixed focation, the tip of the electric vector traces a straight line as a function of time. (From Muller, 1973, with permission from Wiley-VCH.)... 

The angular distributions of the 0(3P2) fragments show the degree of correlation between the product recoil velocity (v) with the electric vector of the dissociating light and are typically characterized by the lab frame anisotropy parameter (/ ) given in the equation,52,53... 

Here 0 is the recoil angle relative to the electric vector of the dissociating light and P2 is the second-order Legendre polynomial. 

Figure 2.10. Direction of electric vector in (a) unpolarized and (b) polarized light the direction of propagation of the light wave is along the horizontal from left to right.

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