A robust comparison of dynamical scenarios in a glass-forming liquid

Abstract

We use Bayesian inference methods to provide fresh insights into the sub-nanosecond dynamics of glycerol, a prototypical glass-forming liquid. To this end, quasielastic neutron scattering data as a function of temperature have been analyzed using a minimal set of underlying physical assumptions. On the basis of this analysis, we establish the unambiguous presence of three distinct dynamical processes in glycerol, namely, translational diffusion of the molecular centre of mass and two additional localized and temperature-independent modes.The neutron data also provide access to the characteristic length scales associated with these motions in a model-independent manner, from which we conclude that the faster (slower) localized motions probe longer (shorter) length scales. Careful Bayesian analysis of the entire scattering law favors a heterogeneous scenario for the microscopic dynamics of glycerol, where molecules undergo either the faster and longer or the slower and shorter localized motions.

Metabasin transitions are Johari-Goldstein relaxation events

“…3 indicates a small activation energy, as expected for a highly local motion. 31,42 The time scales we obtain for the various modes are also consistent with previous work. We 19 and others 30 have previously assigned the intermediate process at τ h ≈ 1 ps to spatially heterogeneous dynamics in the form of collective molecular rearrangements.…”

“…We account for distinct types of motion expected in amorphous systems: over-damped vibration (in which we include IS transitions), motion associated with MB transitions, and homogeneous diffusive motion. Based on ultrafast optical experiments performed by us and on recent work by Vispa et al, 31 we choose Lorentzians to represent each of the three types of motion considered here. Vispa et al, 31 recently found that a triple Lorentzian function provided significantly better fits for S(q, E) of a molecular liquid than common models of similar complexity containing functional forms such as KWW and Gaussian, and much better fits than 2-component models.…”

“…Based on ultrafast optical experiments performed by us and on recent work by Vispa et al, 31 we choose Lorentzians to represent each of the three types of motion considered here. Vispa et al, 31 recently found that a triple Lorentzian function provided significantly better fits for S(q, E) of a molecular liquid than common models of similar complexity containing functional forms such as KWW and Gaussian, and much better fits than 2-component models. In accordance with Vispa's finding, we observe three exponential relaxation processes for propylene carbonate (PC) in time-domain optical Kerr effect data covering similar time and lengthscales to those considered here (manuscript in preparation).…”

“…Although neutron scattering has been applied to liquids and glasses for decades, these signatures have not been identified until now. This is probably because, exceptions notwithstanding, 19,30,31 neutron scattering from molecular liquids has historically been analyzed in terms of homogeneous models 32,33 in spite of overwhelming evidence for short-time DH in these systems.…”

On the microscopic mechanism behind the purely orientational disorder–disorder transition in the plastic phase of 1-chloroadamantane

“…In a second step, the software generates a complete Markov chain containing all the parameters that can describe the data, and their associated χ 2 merit function. The adaptive algorithm and the Bayesian methodology have been described in detail in previous publications [13,[46][47][48][49][50][51][52][53], and the reader is referred to these works for further details.…”