Abstract

Polarization-dependent absorption coefficients of various molecular vibrational bands (i.e., IR markers) in the 2.5–20-μm spectral region were measured for two nematic liquid crystals (LCs), 5CB and MBBA. Combining the measured absorption dichroism of a dipole moment with a correction factor g, which takes the anisotropy of the local field into consideration, the order parameter S of a uniaxial LC can be determined. This g factor is found to have significant effect on the determination of S. The temperature-dependent g factor is calculated for 5CB, for example. Results on S agree well with those obtained by the NMR technique, which does not require the local field correction. Order parameters of several 5CB–host–LC mixtures were measured at room temperature. Results indicate that the LC mixtures with higher nematic-isotropic phase transition temperature exhibit larger S. This phenomenon can be used to improve the LC materials for IR applications.

© 1987 Optical Society of America

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