The Determination of Moments of the Orientational Distribution Function in Liquid Crystals by the Depolarization of Fluorescence of Probe Molecules
The approach to the mathematical formulation of the distribution function that describes long range orientational ordering of molecules in liquid crystals is necessarily an approximation. Some workers propose model forms with one or more adjustable parameters used to fit data of some experimental property sensitive to an average over this unattainable function. These attempts are open to criticism of the reasonableness and uniqueness of the chosen form of the approximation. An alternative procedure, which is mathematically rigorous and unambiguous, is to write the function as a truncated series expansion whose coefficients are experimentally determinable moments of the real distribution. This method is usually limited by the availability of only the second moment. Raman (1–4), electron spin resonance (5), and fluorescent emission spectroscopy (1,2) are capable of supplying both the second and fourth moments, hence extending the expansion and making it a more fiithful representation of the real distribution. The fluorescence technique, however, has inherent complications in that there is usually a significant delay (1–10 nsec) between the absorption and emission process and emitted radiation may emanate polarized along a different axis in the excited molecule.
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