Picosecond Dynamic Heterogeneity, Hopping, and Johari-Goldstein Relaxation in Glass-Forming Liquids

We show that incoherent quasielastic neutron scattering from molecular liquids reveals a two-state dynamic heterogeneity on a 1 ps time scale, where molecules are either highly confined or are free to undergo relatively large excursions. Data ranging from deep in the glassy state to well above the melting point allows us to observe temperature-dependent population levels and exchange between these two states. A simple physical picture emerges from this data, combined with published work, that provides a mechanism for hopping and for the Johari-Goldstein (βJG) relaxation, and allows us to accurately calculate the diffusion coefficient, DT, and characteristic times for α, and βJG relaxations from ps time scale neutron data.