Time-Resolved Light Scattering Studies of Structural Rearrangements in Disordered Condensed Phase Systems.

Structural and orientational relaxation processes in the glass-forming liquid triphenylphosphite (TPP) were studied by impulsive stimulated scattering (ISS), a time -resolved four-wave mixing technique. The 350-150 K temperature range was examined, which covers the transformation from normal liquid to supercooled liquid to glass. Longitudinal and shear acoustic modes were observed and characterized over the frequency range of 20 MHz to 5 GHz. Molecular orientational dynamics are also observed directly, as is a low frequency intramolecular mode of the TPP molecule. A picture of structural and orientational relaxation processes in supercooled TPP consistent with both mode coupling theories and recent theories of structural relaxation processes in supercooled liquids is proposed. Preliminary results of ISS studies of structural relaxation processes in the glass forming liquid salol are also presented. From analysis of the 'time-dependent thermal expansion' mode a distribution of structural relaxation times whose width is independent of temperature is reported. The results are consistent with photon correlation spectroscopy results. Shear acoustic modes from the polymer bisphenol -A polycarbonate are also reported. The longitudinal acoustic anomaly along the a axis of the crystal (N(CH_3)_4] _2ZnCl_4 has been investigated near the normal-incommensurate (N-IC) phase transition temperature by impulsive stimulated scattering, for acoustic frequencies in the range of 350 MHz-4 GHz. Considerable dispersion is seen over this frequency range. A possible explanation for suppression of critical behavior in the acoustic anomaly near the N-IC transition is given. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253 -1690.) (Abstract shortened with permission of school.).