  Recent simulation and model system studies suggest that local structural
excitations play an important role in the dynamics of liquids and glasses.
Here, for the first time, we quantify excitation populations in real liquids,
showing that their temperature-dependent population can be predicted from
entropy and enthalpy of melting. We further show that the excitation population
in the first solvent shell serves as an order parameter for the appearance of
dynamic heterogeneity and for driving transformations between distinct
mechanistic regimes of liquid relaxation. We propose a scenario that provides
simple physical explanations for these previously enigmatic aspects of liquid
behavior.
