A detailed test of mode-coupling theory on all time scales: Time domain studies of structural relaxation in a supercooled liquid

Abstract

The dynamics of supercooled salol (phenyl salicylate) was measured in the time domain using optical Kerr effect techniques. By combining several experimental setups, data spanning more than six decades in amplitude and time (∼100 fs to ∼1 μs) were observed. The data have a complex shape, ranging from high-frequency intramolecular oscillations at short times, to nearly exponential relaxation at long times. As predicted by mode-coupling theory (MCT), the data for some ranges of time appear as power laws. The slowest power law, the von Schweidler power law, has an almost constant exponent of ∼0.59 over the entire temperature range studied (247–340 K). Above the MCT Tc (T>∼1.17 Tg, where Tg is the laboratory glass transition temperature) for t>∼1 ps, the decays are shown to be in excellent agreement with the master curve predicted by ideal MCT when higher order terms are included. However, the data do not display the plateau predicted by ideal MCT. To discuss the data at all temperatures, the intermediate time scale portion of the data, 2<t<10 to 500 ps (depending on the temperature), is modeled as a power law that falls between the critical decay and the von Schweidler power law. This intermediate power law shows significant temperature dependence with an exponent that decreases to a value of ∼−1 below Tc. Calculations using extended MCT, for a full range of hopping times, demonstrate that the temperature dependence of the intermediate time scale data near and below Tc cannot be explained by extended MCT.