Collective excitations in liquid CD4: Neutron scatteringand molecular-dynamics simulations

We have investigated the dynamic structure factor S(Q, ω) ofliquid CD4 at T = 97.7K in the wave vector range 2 ≤ Q/nm−1 ≤ 15 by means ofneutron 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 ofliquid CD4 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 oflinearized hydrodynamic modes. An empirical relation, able to reconcile the different wave vector ranges of mode propagation observed in disparate liquids, is investigated.