ION-DIRECTOR COUPLING IN A FERROELECTRIC LIQUID-CRYSTAL

The coupling between the motion of ionic impurities and the bend-type director distortion in a ferroelectric liquid crystal is examined theo retically. We calculate the relaxation rate of the (fast) director mod e to lowest order in the ion concentration. Moreover, we calculate the ratio K(fast)(tau=0)/K(tau=0), where K(tau) is the amplitude of the t emporal autocorrelation function for director fluctuations, tau is the correlation time, and K(fast)(tau) is the corresponding value for the quickly decaying component of this quantity. The ratio, which can be measured in a light-scattering experiment, yields the inverse Debye-Hu ckel screening length kappa.