The Structure of a Cholesteric Mesophase Perturbed by a Magnetic Field

The application of a magnetic field to a cholesteric mesophase not only aligns the helical structure but also increases the pitch of the helix until at some critical field the cholesteric phase is converted to a nematic mesophase. The theory of this phenomenon has been tested using optical techniques which arehowever unable to probe the structural changes within the pitch of the helix as it is perturbed by a magnetic field. By employing electron resonance spectroscopy we have been able to verify the predictions of the theory concerning the distribution of the director within the perturbed helix of a weakly cholesteric system formed by adding an optically active solute to a nematic mesophase. Further it has proved possible to determine the temperature dependence of the ratio of the twist elastic constant to the anisotropy in the diamagnetic susceptibility for the nematogen, 4,4′-dimethoxyazoxybenzene, used in this study. Finally although the orientational order of the spin probe is found to be the same in both the cholesteric and parent nematic mesophase the extent of angular correlation decreases on the addition of the optically active solute.