Optical dichroism force

The physical processes of absorption, refraction, circular dichroism and optical rotatory dispersion are closely inter-related, such that mechanistic discussions of the latter two require adequate consideration of the former two. Accordingly, this presentation begins with consideration of basic elements and terminologies of absorption and refraction, and then treats circular dichroism and optical rotatory dispersion, and in so doing provides prosthetic group examples of excitation resonance interaction and of the dispersion force interactions giving rise to hyper- and... 

One of the most interesting photoresponsive properties of azo polymers is the photoinduced birefringence and dichroism (Xie et al., 1993). The photoinduced anisotropy is caused by the disparity of the repeated trans-cis isomerization of azo chromophores under linear polarized light irradiation. The most efficient excitation occurs in the polarization direction, which can force the chromophores to continually change their orientation and to be eventually stabilized at the direction perpendicular to the polarization (Natansohn and Rochon, 2002). The effect shows potential applications in areas such as reversible optical data storage, optical switching and sensors. 

In solutions and suspensions of anisotropic molecules directing external forces such as a hydrodynamic flow or electric field forces may cause signal changes 55 originating from molecule rotations. In particular optically and electrically anisotropic (dipolar or polarizable) macromolecules exhibit major electric dichroism and electric birefringence. 

It has to be pointed out that there is a fundamental difference betvi/een both optical axes. The optical axis of a LC-phase vi/ithout crosslinking is a consequence of a specific orientation of the mesogenic molecules in the LC-state and is not caused by external effects. In opposite to this the networks only possess an optical axis if they are deformed by external forces which induce an anisotropy into the network. In this case the optical axis of the network and the axis of deformation are identical. Further information about the orientation of the mesogenic side groups are available by measurements of the linear dichroism which are performed at present. 

While vibrational spectroscopy is not capable of the structural resolution of X-ray diffraction, it nevertheless has some important advantageous features. First, it is not generally limited by physical state samples can be in the form of powders, crystals, films, solutions, membranous aggregates, etc. Second, a number of different experimental methods probe the structure-dependent vibrational modes of the system infrared (IR), Raman (both visible and UV-exeited resonance), vibrational circular dichroism, and Raman optical activity, many of these with time-resolution capabilities. Finally, in addition to providing structural information, vibrational spectra are sensitive to intra- and intennolecular interaction forces, and thus they also give information about these properties of the system. 

The two circular-polarized beams of the incident radiation (I, levorotatory r, dextrorotatory) not only can be influenced with respect to their direction of rotation, but also, in the region of an absorption band, can be absorbed to a different extent by the sample, so that in addition to so-called optical rotatory dispersion (ORD), circular dichroism (CD) or the Cotton effect is observed [35] - (38J. Along with these classical methods of analysis, modem methods for the analytical investigation of surfaces and boundary layers have become very important, including the use of polarized light in ellipsometry ( Surface Analysis) and surface plasmon resonance. Under the influence of external forces (e.g., a magnetic field), even optically inactive substances can be caused to produce magnetooptic rotation. 

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