"de Gennes" narrowing in supercooled molecular liquids : Evidence for center-of-mass dominated slow dynamics

The density correlation function F(q,t) of the two similar substituted aromatic liquids, Toluene and m-Toluidine, is studied by coherent neutron spin-echo and time-of-flight scattering for wave vectors q around the maximum q_max of the total static structure factor S_m(q) in the supercooled i.e. high density state far away from the normal fluid state. The wave-vector dependence of the mean structural relaxation time tau shows in both liquids a very pronounced de Gennes-like narrowing centered around q_0 < q_max, where q_0 corresponds to the first maximum in the center-of-mass static structure factor S_COM}(q). We find that the narrowing can be described quantitatively by using S_COM(q)/q^2 instead of S_m(q)/q^2 indicating that at the corresponding molecular length scales the relaxation of F(q,t) is dominated by purely translational motion.