Optical Study of Shear and Longitudinal Acoustic Waves

The spectroscopic technique Impulsive Stimulated Scattering (ISS) was refined and used to study the complex structural relaxation dynamics of glass forming liquids, allowing both empirical modeling and testing of the predictions of the mode-coupling theory (MCT). Longitudinal and shear acoustic waves throughout much of the MHz frequency range, time-dependent thermal expansion on nanosecond and microsecond scales, and slower thermal diffusion were all monitored in real time. The data were used to construct complex longitudinal modulus spectra spanning from, 30 kHZ to 3 GHz, and complex shear modulus spectra from- 10 MHz to 1 GHz. In the liquid tetramethyl tetraphenyl trisiloxane, experiments which verified time-temperature superposition of its relaxation dynamics permitted construction of a mas-ter plot of scaled relaxation spectra in the entire temperature range studied. MCT predictions of power-law frequency dependencies of the high and low frequency wings of the loss modulus yielded a high-frequency exponent parameter in good agreement with the width of the non-exponential relaxation kinetics. The low-frequency expo