Microscopic relaxation in supercritical and liquid neon

Abstract

The high frequency behavior of the dynamic structure factor, S(Q,ω), of liquid and supercritical neon is investigated by inelastic x-ray scattering at different temperatures and pressure. The spectral evolution is described in terms of a single-relaxation-time viscoelastic model. The occurrence of a positive dispersion in the sound velocity is clearly visible in both investigated thermodynamic phases. The anomalies in the dispersive behavior deeply reduce at the higher temperatures, probably, as a consequence of important changes in the first shell interactions. More generally, the atomic dynamics is dominated by a relaxation process whose time scale is in the range of fast microscopic degrees of freedom (≈10−13 s), and whose strength and typical time scale stay constant over all the explored liquid and supercritical regions.

High-frequency dynamics of liquid and supercritical nitrogen

“…As already observed in other experiments performed in this Qrange [6][7][8][9][20][21][22], we found that the value of τ α (Q) decreases with increasing Q at a given temperature. As an example we report the Q-dependence of such a parameter for two selected temperatures in the inset of fig.3.…”

“…In the intermediate (visco-elastic) regime the acoustic propagation is strongly coupled with the active relaxation. The sound velocity strongly depends on the frequency of the probed acoustic wave, reflected in an upwards bending of the sound dispersion relation with respect to the linear behaviour characteristic of the viscous regime (positive sound dispersion) [4][5][6][7][8][9].…”

Collective modes and thermodynamics of the liquid state

“…Experiments have shown that in the highly viscous regime just above T g , C p measured at high frequency and representing the vibrational part of heat capacity coincides with the total low-frequency heat capacity usually measured [126,127], consistent with Eq. (74). In the glass transformation range close to T g , the two heat capacities start to differ due to non-equilibrium effects and freezing of configurational entropy, and coincide again below T g in the solid glass.…”

“…Following the detection of PSD above the critical point [74,116], the Widom line was proposed to separate two supercritical states where PSD does and does not operate [75] (see [111] for the discussion of extrapolating the line to high pressure and temperature where no maxima exist). The states with and without PSD were called "liquidlike" and "gas-like" because they resemble the presence and absence of PSD in subcritical liquids and gases.…”

Collective dynamics in noble-gas and other very simple classical fluids

“…Later, more x-ray data at a number of thermodynamic states of gaseous and liquid neon were analyzed using, instead, the SVE model [52]. Here, a positive dispersion present in the liquid also tends to disappear in approaching supercritical temperatures.…”