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

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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 qₘax of the total static structure factor Sₘ(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 q₀ < qₘax, where q₀ corresponds to the first maximum in the center-of-mass static structure factor SCOM(q). We find that the narrowing can be described quantitatively by using SCOM(q)/q² instead of Sₘ(q)/q² indicating that at the corresponding molecular length scales the relaxation of F(q,t) is dominated by purely translational motion.