IMPULSIVE STIMULATED SCATTERING STUDY OF NORMAL AND SUPERCOOLED LIQUID TRIPHENYLPHOSPHITE

We report the results of a study of the glass-forming liquid triphenylphosphite over the temperature range 350 to 150 K by impulsive stimulated scattering, a time-resolved four-wave mixing technique. The temperature range examined covers the transformation from normal liquid to supercooled liquid to glass. The first time-resolved observations of propagating shear acoustic waves in a liquid are reported, over the frequency range of 120 MHz to 1.7 GHz. Longitudinal acoustic waves are observed and characterized over the frequency range of 20 MHz to 5 GHz. Molecular orientational dynamics are also observed directly, as is a slow density response to heating (time-dependent thermal expansion). The longitudinal acoustic data are analyzed phenomenologically in terms of a distribution of relaxation times. Although no commonly used distribution is found to be completely adequate, the Cole-Cole distribution is used and yields average relaxation times tau-L which follow a Vogel-Tamann-Fulcher law with T(o) = 200 K, and a width which increases dramatically as the temperature is lowered. The slow density response to heating is found to follow a single exponential rise to the rise time tau-M consistently larger than tau-L. These facts are taken as evidence that the distribution of relaxation times which couples to density changes may have some degree of structure, and may be bimodal or multimodal in nature. The orientational motion is found to follow an Arrhenius law, in contrast to earlier results, with orientation times tau-or which are very close to tau-M. The shear wave behavior is also analyzed in terms of a Cole-Cole distribution of relaxation times, which at all temperatures appears to be narrower than the distribution which couples to the longitudinal waves.