Angular dependent linewidths for a spin probe dissolved in a liquid crystal

Abstract

A spin probe dissolved in the mesophase of a nematic or smectic liquid crystal behaves, in many ways, as if it were encorporated in a uniaxial single crystal. For example the positions of the lines in the electron resonance spectrum of the probe depend on the orientation of the liquid crystal with respect to the magnetic field. In addition the widths of the spectral lines might also be expected to be angular dependent. The form of this angular dependence is readily calculated provided the dominant spin relaxation process results from molecular reorientation with respect to the director in the mesophase. In this paper we develop a theory for the angular dependence of the linewidths and show how it could be used, with some advantage, to investigate molecular reorientation in a macroscopically anisotropic system. The observed angular dependence of the linewidths for a nitroxide spin probe dissolved in the mesophase of a smectic A liquid crystal confirms certain aspects of the theory.