PHOTOBLEACHING TECHNIQUE FOR MEASURING ULTRASLOW REORIENTATION NEAR AND BELOW THE GLASS-TRANSITION - TETRACENE IN O-TERPHENYL

A photobleaching technique for measuring probe rotational diffusion in supercooled and glassy materials is presented. Rotational correlation times from 10(-2) to 10(+4.5) s have been obtained for tetracene in o -terphenyl (OTP). Relative to the conventional T(g) for OTP (-30-degre es-C), this corresponds to a temperature range of T(g)+ 15-degrees-C t o T(g) - 5-degrees-C. These results, combined with nanosecond measurem ents of anthracene rotation in OTP, are compared to OTP viscosity data . We find that, in contrast to translational diffusion measurements of probes in OTP, rotational diffusion follows the temperature dependenc e of eta/T over the 14 decades of time and viscosity explored. We also explore several possible experimental artifacts associated with the p hotobleaching technique. We find no evidence that any artifacts influe nce the results under typical operating conditions.