MOLECULAR-DYNAMICS IN PERFLUORO-N-EICOSANE .4. OSCILLATORY AND DIFFUSIVE ROTATIONAL MOTION

Rotations of the molecules about their long axis constitute an importa nt part in the complex pattern of internal motions in the rhombohedral phase of perfluoro-n-eicosane (C20F42). Dynamical studies were perfor med by quasielastic and inelastic neutron scattering. Spectra were det ermined in a range of strong diffuse scattering. They show a superposi tion of librations (deltaE congruent-to 1-2 meV) and rotational diffus ion (deltaE congruent-to 10-100 mueV). Reorientation rates exhibit a s lowing down on approaching the transition to the low temperature phase (T(c) = 200 K). As indicated by this slowing down, by the nonexponent ial relaxation function, and also an intensity enhancement at the Brag g-positions, rotations occur correlated between neighboring molecules. The state of motion can be simulated by molecular dynamics calculatio ns performed for a two-dimensional lattice of coupled rotators.