Dynamics of the rotational degrees of freedom in a supercooled liquid of diatomic molecules

TL;DR

This study uses molecular dynamics simulations to explore rotational dynamics in supercooled diatomic liquids, revealing critical temperature behavior, deviations from theoretical principles, and the significance of molecular flips at low temperatures.

Contribution

It provides new insights into rotational relaxation and diffusion in supercooled liquids, highlighting deviations from mode-coupling theory and the role of molecular flips.

Findings

Relaxation times follow a power-law near critical temperature T_c.

Rotational diffusion constant D_r shows Arrhenius behavior in supercooled regime.

180-degree flips significantly influence low-temperature rotational dynamics.

Abstract

Using molecular dynamics computer simulations, we investigate the dynamics of the rotational degrees of freedom in a supercooled system composed of rigid, diatomic molecules. The interaction between the molecules is given by the sum of interaction-site potentials of the Lennard-Jones type. In agreement with mode-coupling theory (MCT), we find that the relaxation times of the orientational time correlation functions C_1^(s), C_2^(s) and C_1 show at low temperatures a power-law with the same critical temperature T_c, and which is also identical to the critical temperature for the translational degrees of freedom. In contrast to MCT we find, however, that for these correlators the time-temperature superposition principle does not hold well and that also the critical exponent gamma depends on the correlator. We also study the temperature dependence of the rotational diffusion constant D_r and demonstrate that at high temperatures D_r is proportional to the translational diffusion constant D and that when the system starts to become supercooled the former shows an Arrhenius behavior whereas the latter exhibits a power-law dependence. We discuss the origin for the difference in the temperature dependence of D (or the relaxation times of C_l^(s) and D_r. Finally we present results which show that at low temperatures 180 degree flips of the molecule are an important component of the relaxation dynamics for the orientational degrees of freedom.