Collective excitations in liquid CD 4 : Neutron scattering and molecular-dynamics simulations

We have investigated the dynamic structure factor S ( Q ,ω) of liquid CD 4 at T = 97.7 K in the wave vector range 2 ⩽ Q /nm −1 ⩽ 15 by means of neutron scattering and molecular-dynamics simulation, in order to study the centre-of-mass collective dynamics. The agreement between the experimental spectra and those simulated using a recent ab initio based intermolecular potential is good, particularly at low Q . Underdamped collective excitations, detected in the whole experimental Q -range, characterize the dynamics of liquid CD 4 as markedly different from that of other molecular liquids. Also, the energy and damping of collective excitations in methane are shown to differ considerably, even at the lowest measured Q -values, from those of linearized hydrodynamic modes. An empirical relation, able to reconcile the different wave vector ranges of mode propagation observed in disparate liquids, is investigated.