Polarized light, structural analysis methods using

The early speculations about transition moment directions in the lower aromatics needed to be tested experimentally. The traditional vapour phase measurements of band lineshapes were not accessible with the spectroscopic resolving power we then had, but came later (sec 4.2). Most absorption systems of aromatics in the UV were in any case diffuse and structureless. The method that could be used was to get the polarization of the light absorbed by the crystal composed of the target molecules. The positions and orientations of the molecules in the laboratory frame were known from X-ray crystal structure analysis. The interpretation of results raised non-trivial theoretical problems of its own. The experiments will be described first. 

Both molecular and transition dipole moment orientation can be probed within the solid state samples, especially upon combining structural information with polarized absorption measurements. Small-area electron diffraction experiments are also effective since they allow the orientation of crystalline regions within polymer nanofibers to be probed. Most of these techniques are already well established from the study of polymer alignment in thin-films. Improved analysis methods, which make use of combined polarized Raman spectroscopy and UV-visible absorption data, are especially worthwhile to be mentioned as valuable tools to investigate the orientational properties of light-emitting polymer systems. We will come back in depth to optical properties of polymer nanofibers in Chapter 5. 

The solid-state properties like crystallinity, polymorphism (crystal structure), shape (morphology), and particle size of drugs are important in the stability, dissolution, and processibility of drugs. Some commonly used methods in solid-state studies include microscopy, hot stage microscopy with polarized light, x-ray powder diffraction (XRPD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared FTIR/Raman, and solid-state NMR. 

A sensitive approach for the analysis of molecular structures is the measurement of rotation time constants and optical anisotropy coefficients by relaxation electrooptical methods.The experimental procedure used for electrooptical investigations is relatively simple samples are subjected to electric field pulses, and the response due to field-induced alignment or field-induced reactions is recorded by spectrophotometric techniques. In the dichroism experiments, the absorbance of polarized light is measured under electric field pulses. The measured quantity, linear dichroism (LD), means that anisotropic absorption of plane or linearly polarized light has taken place. 

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