Molecular motions in a smectic a phase by incoherent quasi' elastic neutron scattering

Results are presented of incoherent quasi-elastic neutron-scattering measurements in ethyl 4-(4’acetoxybenzylidene)aminocinnamate (EABAC) in crystal, S_a and nematic phases. Discussion is concentrated on data for the S_a phase of an aligned, tail deuteriated, sample for Q ┴n and Qn, where Q is the scattering vector and n the direction of the unique axis (the layer normal). The scattering law S(Q_w) is found to be essentially identical for the two directions. Analysis of the results shows that the molecules undergo rapid reorientation about the long axes with D_r˜ 3 x 10¹⁰ rad s^-1, together with a random but bound motion predominantly normal to the layers with an r.m.s. amplitude <Z²>^1/2˜ 1.2-1-8 Å, depending on the model, and a correlation time ˜ 10^-11 s.