Relaxation and self-diffusion of supercooled liquids derived from picosecond timescale dynamics

We use neutron scattering and nonlinear optical measurements to investigate ps-ns timescale dynamics in liquid, supercooled liquid, and glassy states. The experimental observables show evidence of dynamic heterogeneity on this timescale that supports a facilitated dynamics picture. We obtain a direct measure of the concentration of molecular excitations, or mobile regions, as a function of time and temperature. Using a model [1] broadly consistent with that proposed by Chandler and co-workers [2], we are able to quantitatively predict self-diffusion rates and Stokes Einstein violation deep in the supercooled regime directly from ps timescale and Angstrom - nanometer length scale measurements for all systems we have investigated. The model we employ also provides a clear physical mechanism for the Johari-Goldstein relaxation process. [1] M.T. Cicerone, Q. Zhong & M. Tyagi, PRL 113 117801 (2014). [2] J. P. Garrahan & D. Chandler, Coarse-grained microscopic model of glass formers, PNAS 100, 9710 (2003).