How long do regions of different dynamics persist in supercooled o-terphenyl?

In supercooled o-terphenyl (OTP), subsets of probe molecules in more mobile environments can be selectively photobleached. The time required for the remaining slower-than-average probes to be redistributed into an equilibrium set of environments has been measured. At T_g + 4 K (T_g = 243 K), this exchange time is 6 times greater than the average probe rotational correlation time. These results are compared to previous optical measurements at T_g + 1 K (Cicerone, M. T.; Ediger, M. D. J. Chem. Phys. 1995, 103, 5684), which showed that the exchange time is more than 100 times the rotational correlation time, and to multidimensional NMR experiments on deuterated OTP at T_g + 10 K (Bohmer, R.; et al. Europhys. Lett. 1996, 36, 55), which showed that the exchange time is nearly equal to the correlation time. These results in aggregate suggest that a new relaxation process in equilibrium supercooled liquids emerges only at temperatures very near T_g. A model for selective photobleaching is proposed. The model reproduces the experimental data reasonably well and indicates that the photobleaching efficiency of a given probe is only weakly correlated with its rotational correlation time.