Incoherent cold neutron scattering investigation of stochastic molecular orientational fluctuations in liquid crystals

The model of rapid orientational fluctuations of the molecular long axis in liquid crystals, based upon the solution of the one-dimensional problem of the random walk of a particle between two perfectly reflecting barriers, is suggested and in conjunction with the translational diffusion of the molecular centre of mass and also uniaxial rotational diffusion, utilized for the evaluation of the incoherent scattering function, as well as EISF, appropriate to smectic A and nematic phases, and the comparison with measurements is made. On the basis of published measurements it is concluded that the molecules in the smectic A phase of DTBBA are subject to long axis orientational fluctuations between two potential barriers with an apex angle of about 100°.