HELICAL MOTIONS OF ALIPHATIC CHAINS IN MOLECULAR-CRYSTALS - THE INCOHERENT QUASI-ELASTIC NEUTRON-SCATTERING LAW

The incoherent quasi-elastic neutron scattering (IQNS) law for the res tricted diffusive motion of a particle along a spiral (helical motion) is derived in order to model correlated rotational and translational motions of molecules in the dynamically disordered phases of organic m olecular crystals. In addition, we present a unified formulation to ca lculate the scattering laws for restricted translational diffusion, un iaxial restricted rotational diffusion and uncorrelated restricted tra nslations and rotations. We therefore discuss the various experimental geometries needed to analyse in detail these motions. Spectra are sim ulated and analysed on the basis of IQNS experiments performed to stud y the dynamics of n-nonadecane molecules in various crystalline enviro nments. We also discuss the existence of correlated and uncorrelated r otational and translational motions in the 'rotator' phase of pure n-n onadecane samples.