An electron resonance investigation of molecular motion in the smectic A mesophase of a liquid crystal

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Abstract
This paper reports the angular dependence of the electron resonance spectrum of a spin probe dissolved in a smectic A mesophase. The spectra were analysed to obtain the angular variation of the linewidth coefficients and hence the angular linewidth coefficients. These angular coefficients may be interpreted by employing a theory of spin relaxation based on the strong collision model for molecular reorientation. Such an analysis yields values for the correlation times for reorientation about the long and short axes of the spin probe as well as the magnitude of the solute order parameter [Pbar] 4. The possibility of interpreting the angular linewidth coefficients in terms of a theory based on the diffusion model for molecular reorientation is also explored. Relatively straightforward expressions for these coefficients can only be obtained from the diffusion model when the orientational order is high, as it is in a smectic phase. The analysis of the angular linewidth coefficients then provides values for the components of the diffusion tensor parallel and perpendicular to the long axis of the spin probe. The large anisotropy in the diffusion tensor may be indicative of the unhindered molecular motion about the long axis. Both models of molecular reorientation in a highly ordered system appear to provide a satisfactory account of the observed angular linewidth coefficients.