Diffusive motions in liquid medium-chain n -alkanes as seen by quasielastic time-of-flight neutron spectroscopy

Different diffusive motions in liquid C32 H66 on a picosecond time scale could be disentangled by resolution resolved quasielastic time-of-flight neutron spectroscopy (QENS). It is demonstrated that at all observation times, the dominating motion causes a Q2 proportionality of the QENS signal, which indicates a Fickian diffusion mechanism. The observed motions can be characterized by an observation time dependent apparent diffusion coefficient Da (to), which is up to one order of magnitude larger than the molecular self-diffusion coefficient Ds. By comparison with molecular dynamics simulations, the identified motions are attributed to displacements of hydrogen atoms reflecting not only global but also local molecular trajectories. Despite the rodlike shape of the molecules, the center of mass diffusion was found to be essentially isotropic. A coherent picture of the diffusional processes ranging from the fast tumbling of CH2 groups to the slow long range molecular diffusion is presented.