Molecular-Dynamics in Perfluoro-n-Eicosane. 4. Oscillatory and Diffusive Rotational Motion

Abstract

Rotations of the molecules about their long axis constitute an important part in the complex pattern of internal motions in the rhombohedral phase of perfluoro-n-eicosane (C20F42). Dynamical studies were performed by quasielastic and inelastic neutron scattering. Spectra were determined in a range of strong diffuse scattering. They show a superposition of librations (deltaE congruent-to 1-2 meV) and rotational diffusion (deltaE congruent-to 10-100 mueV). Reorientation rates exhibit a slowing down on approaching the transition to the low temperature phase (T(c) = 200 K). As indicated by this slowing down, by the nonexponential relaxation function, and also an intensity enhancement at the Bragg-positions, rotations occur correlated between neighboring molecules. The state of motion can be simulated by molecular dynamics calculations performed for a two-dimensional lattice of coupled rotators.