Propagation of acoustic excitations in a liquid at large wavevectors: a molecular-dynamics study

A two-decade development of dedicated high-performance instrumentation at major neutron and synchrotron radiation sources has stimulated significant growth in the number and quality of spectroscopic studies of the collective dynamics of fluids, making Brillouin scattering, in its neutron and x-ray ‘versions’, a fast growing field of research. However, in contrast with the large amount of work done for wavevector Q lower than the position Qp of the main peak of the static structure factor S(Q), very little is known about the behaviour of acoustic excitations at much larger Q. We present molecular-dynamics simulation results for the translational part of the dynamic structure factor of the molecular liquid CD4 up to high Q values (Q ∼ 4Qp), analysed through the fitting of the viscoelastic model line shape. The analysis, carried out by applying the concepts described in a recent paper, shows that underdamped sound excitations persist at least up to such high Q values, in agreement with the existence of the distinct part of S(Q), with the exception of a restricted interval around Qp where the collective oscillations become overdamped