Relaxation of spatially heterogeneous dynamic domains in supercooled ortho-terphenyl

Abstract

A photobleaching technique has been used to observe rotational dynamics of dilute probe molecules in supercooled o-terphenyl (OTP). The nonexponential rotational relaxation of the probe molecules is shown to be due, at least in part, to the presence of spatial heterogeneity in the host dynamics. Under appropriate photobleaching conditions, a nonequilibrium distribution of probe molecule mobilities can be created by preferentially bleaching the more mobile probe molecules in a sample. Near Tg this nonequilibrium distribution is observed to return to an equilibrium distribution of relaxation times over times on the order of 103 τc, where τc is the average rotational correlation time of a probe molecule. The time required to return to equilibrium is interpreted as a structural relaxation time for dynamic heterogeneities in OTP.