Localization-delocalization transition of the instantaneous normal modes of liquid water

On the temperature dependence of the density of states of liquids at low energies

We report neutron-scattering measurements of the density of states (DOS) of water and liquid Fomblin in a wide range of temperatures. In the liquid phase, we confirm the presence of a universal low-energy linear scaling of the experimental DOS as a function of the frequency, g ( ω ) = a ( T ) ω , which persists at all temperatures. The low-frequency scaling of the DOS exhibits a sharp jump at the melting point of water, below which the standard Debye's law, g ( ω ) ∝ ω 2 , is recovered. On the contrary, in Fomblin, we observe a continuous transition between the two exponents reflecting its glassy dynamics, which is confirmed by structure measurements. More importantly, in both systems, we find that the slope a(T) grows with temperature following an exponential Arrhenius-like form, a ( T ) ∝ exp ( - ⟨ E ⟩ / T ) . We confirm this experimental trend using molecular dynamics simulations and show that the prediction of instantaneous normal mode (INM) theory for a(T) is in qualitative agreement with the experimental data.

Long-range correlation of intra-molecular and inter-molecular vibration in liquid CCl4

Experiments measuring the polarization dependence of hyper-Raman light scattering reveal long-range correlation of molecular vibrations in liquid CCl₄. The ν₃ and ν₁ + ν₄ intra-molecular vibrations at about 770 cm^-1 are strongly polarized transverse to the scattering wavevector. Weaker transverse polarization is exhibited by the ν₁, ν₂, and ν₄ intra-molecular vibrations and by the inter-molecular collision-induced band around 0 cm^-1. The observed polarization dependence is due to the correlation of the vibrations on molecules separated by about 200 nm. The strength of the observed correlation increases with the transition dipole moment for the vibration mode and is consistent with dipole-dipole coupling.