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-degrees-C), this corresponds to a temperature range of T(g)+ 15-degrees-C to T(g) - 5-degrees-C. These results, combined with nanosecond measurements 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 dependence of eta/T over the 14 decades of time and viscosity explored. We also explore several possible experimental artifacts associated with the photobleaching technique. We find no evidence that any artifacts influence the results under typical operating conditions.