Time Domain Optical Studies of Dynamics in Supercooled o-Terphenyl: Comparison to Mode Coupling Theory on Fast and Slow Time Scales

Orientational relaxation of supercooled o -terphenyl (OTP) is studied over a wide range of temperatures and time scales using optical heterodyne detected optical Kerr effect techniques. A combination of experimental setups made it possible to study the dynamics for greater than six decades of time (100 fs to hundreds of nanoseconds). The complex shape of the data is analyzed in terms of power laws at intermediate times and an approximately exponential decay at long times. Most features of the relaxation curves are in accord with predictions of mode-coupling theory (MCT) for supercooled liquids. In particular, at long times the relaxation data show a temperature independent shape in the temperature regime 361−290 K. When plotted vs temperature, the long time decay constants scaled by a relationship given by MCT fall on a line as predicted and give a value of the MCT critical temperature, T c = 285 K, in accord with literature values. On a faster time scale, the slowest of the power law decays has a temperature-independent exponent, which is also in agreement with the predictions of MCT. Detailed comparisons of the data to MCT “master curves” show excellent agreement at times >10 ps. However, the faster component of the data (<10 ps) somewhat deviates in a systematic manner from the ideal MCT predictions. The deviations are in contrast to recent experiments on salol, 1 in which the predictions of ideal MCT reproduced the temperature-dependent data down to T c on all time scales. Comparisons of the OTP data to calculations based on extended MCT do not improve the agreement. It is noted that a portion of the data at intermediate times (faster than the slowest power law), 2 ps < t < 5 ps to 300 ps (depending on temperature) can be described by a power law with a temperature-dependent exponent.